Variants with different assets

We are planning to use variants to build assets for different locales/resolutions/platforms. However on adding different assets to the variants, the AssetBundle Browser complains that
"variants of a given bundle must have exactly the same asset between them based on a name.extension (without Path) comparison"

However the bundles still seem to get built successfully. What is the reason for this limitation? Is this a hard limitation in Unity, or can this error be safely ignored?