Build Report question

Hi guys.
We have a question about Build Report.

I have created some test project which contains only 2 images.
Image1.gif
Imga2.gif

And when I build project and open Build Report from Editor Log I get following result.

Build Report

Uncompressed usage by category:
Textures 654.5 kb 12.4%
Meshes 0.0 kb 0.0%
Animations 0.0 kb 0.0%
Sounds 0.0 kb 0.0%
Shaders 0.0 kb 0.0%
Other Assets 2.0 kb 0.0%
Levels 5.3 kb 0.1%
Scripts 599.6 kb 11.3%
Included DLLs 3.9 mb 76.0%
File headers 9.0 kb 0.2%
Complete size 5.2 mb 100.0%

Used Assets and files from the Resources folder, sorted by uncompressed size:
626.0 kb 11.8% Assets/Images/Image1.gif
29.8 kb 0.6% Assets/Images/Image2.gif
4.1 kb 0.1% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/GUISystem/UnityEngine.UI.dll
4.1 kb 0.1% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/GUISystem/Standalone/UnityEngine.UI.dll
1.8 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Networking/UnityEngine.Networking.dll
1.8 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Networking/Standalone/UnityEngine.Networking.dll
1.5 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Timeline/RuntimeEditor/UnityEngine.Timeline.dll
1.5 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Timeline/Runtime/UnityEngine.Timeline.dll
0.7 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/UnityHoloLens/RuntimeEditor/UnityEngine.HoloLens.dll
0.7 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/UnityHoloLens/Runtime/UnityEngine.HoloLens.dll
0.4 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/TestRunner/UnityEngine.TestRunner.dll
0.3 kb 0.0% Resources/unity_builtin_extra
0.3 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Advertisements/UnityEngine.Advertisements.dll
0.1 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/UnityAnalytics/UnityEngine.Analytics.dll
0.1 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/UnityPurchasing/UnityEngine.Purchasing.dll



So everything looks clear here. Two images
626.0 kb 11.8% Assets/Images/Image1.gif
29.8 kb 0.6% Assets/Images/Image2.gif

But when I create SpriteAtlas and add this to images to it build report is something different and unclear.

Uncompressed usage by category:
Textures 1.0 mb 18.1%
Meshes 0.0 kb 0.0%
Animations 0.0 kb 0.0%
Sounds 0.0 kb 0.0%
Shaders 0.0 kb 0.0%
Other Assets 2.3 kb 0.0%
Levels 5.3 kb 0.1%
Scripts 599.6 kb 10.6%
Included DLLs 3.9 mb 71.1%
File headers 9.0 kb 0.2%
Complete size 5.5 mb 100.0%

Used Assets and files from the Resources folder, sorted by uncompressed size:
1.0 mb 18.1% Built-in Texture2D: sactx-512x512-fmt4-0e3a54370199b696ca539def25218f53
4.1 kb 0.1% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/GUISystem/UnityEngine.UI.dll
4.1 kb 0.1% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/GUISystem/Standalone/UnityEngine.UI.dll
1.8 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Networking/UnityEngine.Networking.dll
1.8 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Networking/Standalone/UnityEngine.Networking.dll
1.5 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Timeline/RuntimeEditor/UnityEngine.Timeline.dll
1.5 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Timeline/Runtime/UnityEngine.Timeline.dll
0.9 kb 0.0% Assets/Images/Image1.gif
0.7 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/UnityHoloLens/RuntimeEditor/UnityEngine.HoloLens.dll
0.7 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/UnityHoloLens/Runtime/UnityEngine.HoloLens.dll
0.4 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/TestRunner/UnityEngine.TestRunner.dll
0.4 kb 0.0% Assets/Images/Image2.gif
0.3 kb 0.0% Resources/unity_builtin_extra
0.3 kb 0.0% Assets/Images/atlas.spriteatlas
0.3 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/Advertisements/UnityEngine.Advertisements.dll
0.1 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/UnityAnalytics/UnityEngine.Analytics.dll
0.1 kb 0.0% /Applications/Unity/Unity.app/Contents/UnityExtensions/Unity/UnityPurchasing/UnityEngine.Purchasing.dll


There are these four files in report which are unclear to me.
If we've created sprite atlas why should we have image1 and image2 textures included in build?
And why their size is different from first build's texture sizes?
And what is Built-in Texture2D ?

1.0 mb 18.1% Built-in Texture2D: sactx-512x512-fmt4-0e3a54370199b696ca539def25218f53
0.3 kb 0.0% Assets/Images/atlas.spriteatlas
0.4 kb 0.0% Assets/Images/Image2.gif
0.9 kb 0.0% Assets/Images/Image1.gif


Any help would be appreciated.
Thanks.