I get this too & all my pans are straight sided- I threw away all those anoying flared ones years ago. I have finally figured that for mine it must be an issue of drying out as the cakes cool. If I leave the cake in the tin, even for a short while, it will be smaller around the top- probably because the moisture can evaporate from the top so it shrinks inwards. I have just wrapped a 10" round that I took out of the oven an hour ago, I took it out of the tin & removed the lining paper straight away then left it to cool on a rack with a clean towel over the top & the sides on this one are straight. It worked for the two cakes for the 8" tier I baked earlier too. I will just have to put up with handling hot cakes & pans & peeling off hot steamy cake liners if it means I don't have to faff around getting all the edges straight.
The tins are straight sided (I measured) so I will try and change what I use to grease the tin and also try to remove the cakes from the tins after baking and before covering them and see if that helps!