time machine backup

i have an imac with 3 accounts. I started using time machine on one account, And this was working fine.

The backup was put on a synology nas. After a few weeks i switched to one of the other accounts and i saw that time machine was making a full backup for that account. I was thinking that if time machine is working on one account and making backup's he does that for all the accounts. But it looks like you have to choose every account to start a time machine backup, or am i wrong ??

I reserved 1 Tb on my NAS to make backup's for the MAC

Thank

Remco