Here's how it works.
Admins can change a post's rating by +5 or -5.
Regular members can change a post's rating by +2 or -2.
The same member/Admin cannot rate multiple times.
An Admin/member cannot...
more... vote on their own posts.
So what you saw is OK. When you saw a posts rating change from -5 to -3, it just means that an Admin first rated it -5 and then a regular member rated it +2 - so -5+2 = -3.
We ask that all Admins and members rate every post they read. It just takes a moment. This way no one feel that they are unfairly rated. If just 2 or 3 people are active in rating, then one might feel that someone is targeting them. If on average, about 50 people rate every post, then the ratings will more likely reflect the general consensus.
Thanks.