Lose precision on IOS Metal.

I tried to test a simple scene exported from Unity3D in my iPhone. However, I find that some float4 variables are automatically converted to half4.

This is my shader code in Unity:

Code (CSharp):

1. struct VertexInput
2. {
3.     float4 vertex : POSITION;
4.     half3 normal : NORMAL;
5.     half4 tangent : TANGENT;
6.     float2 uv0 : TEXCOORD0;
7.     float2 uv1 : TEXCOORD1;
8.  
9. #if defined(USE_UV2)
10.     float2 uv2 : TEXCOORD2;
11. #endif
12.  
13. #if defined(USE_TREE_ANIMATION)
14.     float4 uv2 : TEXCOORD2;
15.     float4 uv3 : TEXCOORD3;
16. #endif
17.  
18.  
19. #if defined(USE_VERTEX_COLOR)
20.     fixed4 vertexColor : COLOR;
21. #endif
22.  
23.     UNITY_VERTEX_INPUT_INSTANCE_ID
24. };

And when I test my scene in XCode:

Why does Metal convert my float4 type to half4?