I play under VH and sometimes even if you vote in your own pope when you had a low relation with the papal states I only got up to 2 below the max. Most of the time I do get the max, but also if I vote in my ally's cardinal.

The entire point is, that the person whose pope gets elected gives near perfect relations to all voters and a bonus for those who either a) own the cardinal or b) are allies of the nation whose cardinal was elected.

Example: Hungary and Portugal are the only other cardinals out of the 12 cardinals in the college. The other 10 are mine. Portugal is my ally and Hungary my enemy. Pope dies and mine gets elected. Both others vote for their own pope. However, Portugal ends up with perfect relations (they only had 3 crosses before) and Hungary is down to like 4 crosses.

So yes, my conjecture does make sense. You also do get excommed by your own pope at times btw, at the same rate another pope you voted for would.