How does heirloom mechanism work?

I'm a little bit confused about how retirement and work.

I 've retired 5 sims so far and FOUR of them (actually two for each heirloom) gave me the same heirloom so I only have 3 individual heirlooms from retirement sofar like WTH EA !!!?

Each of them had different jobs and hobbies and some overlapping traits in case you are wondering.

Also, does a sim's achievement has any effect on an heirloom you will recieve after retirement at all??

Coz one of my sim I retired maxed out both hobby and her job and yet she only gave me Lv.1 Violet's heirloom.