This needs to be debugged to see where the data discrepancy comes from.
From UDN
https://udn.unrealengine.com/s/case/5004z00001mAW14AAG/how-to-properly-access-screen-data-by-uv-coordinate?fromCase=1
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-178985 in the post.