frame by frame sharper than rendered output

So this is peculiar.

I have project with a lot of compound clips and scaling going on. Mostly it's all working well, but I've noticed that on several of the clips, they are soft on playback. BUT if I go through the timeline frame-by-frame with the arrow key, they are noticeably sharper. as soon as i stop, the current frame goes from sharp to soft. This would not bother me except that it seems the soft version is what I'm getting on export. Does this make sense to anybody as to why this might be? Why is it rendering well frame by frame on-the-fly and then poorly in export?