The start center has some rows and columns depending on the available height/width. When changing the size, these rows are taken as basis for the thumbnail position which move up/down consequently. It is more common and less noisy to extend the white space at the end and keep the thumbnail position static.
It doesn't really bother me, but will be interesting to see how it looks like without the movement -> NEW
Allowing the MRU thumbnail views to move around (their packing wraps) in response to resizing the SC backing panel has worked well since inception. Don't see much need to somehow hold position of thumbnails when packing the UI after a resize. IMHO => WF