the problem you have is assuming that each of your characters would have the same enemy list and in the same order.
the enemy list for each of your characters is essentially random and can reset after a cast.

I have had 3 targets and used tab to select different targets and
( start my targeting first target either manually or with tab )
/cast xxx
tab
/cast xxx
tab
???? target is not the 3rd target but the first target all over again
tab
Might be 2nd target again, might be 3rd target.

I think this is because the second cast reset my target list and order of targets or they might have moved relative to how close they are to me, the result is the same though that the target list obtained with tab is in a different order after the second cast.