Looked at this a bit with ericrk@ today. Its most likely the fact that the GPU image decode cache only caches scaled decodes (both discardable and uploads) in the sync decode path. With checker-imaging we decode and lock the original decode forcing the cache to use the original decode instead.

I'm going to try changing checker-imaging to request decodes at the requested scale instead (https://codereview.chromium.org/2928433003/) and see if that fixes it.