Players do get ejected with the intended location priority. However, mobs get ejected with the following priority:
Right, Front, Back, Left front, Right front, Left back, Right back, Left, Centre

This behaviour seems consistent relative to the minecart movement but I am guessing it is not the intended behaviour. It should be Right, Left, Back Right, Back Left, Front Right, Front Left, Back, Front as stated here:
https://bugs.mojang.com/browse/MC-51987?focusedCommentId=251145&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-251145.

It is also worth mentioning that solid blocks 1 or 2 blocks high relative to the activator rail are considered valid locations for mobs, unlike for players (this was only true for 1.11-, Mojang actually removed the useful 'bug' while the annoying one still remains. In my opinion this was a useful feature that made easier the placement of mobs).