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).
Comments 8
Per MC-51987 (this comment) it should be Right, Left, Back Right, Back Left, Front Right, Front Left, Back, Front
Well, I tested it again in 1.10.2 and also in 16w33a in different worlds/loacations and, again, I obtained with no exceptions the same results mentioned above, so I really have no idea why it would work ok for you. Have you tested this in 1.9.4? Could someone else give their feedback on this?
"It is also worth mentioning that solid blocks 1 or 2 blocks high relative from the activator rail are considered valid locations for mobs, unlike for players." This was changed in 1.9.3. It was a bug