Interesting responce from ebay about why my feedback was 645 and I have 750 positive commits. It seems that you can only get one positive feedback per buyer or seller per week. So, if you buy three items from one seller and he gives you three positive feedbacks you only get credit for one in the same week. Never knew this !

Here's the answer from ebay:

Please understand that, members can affect each other's Feedback Score
by only one point per week. Each member may affect your score by one
point per transaction. So if you're leaving feedback to the seller for
the five items you've bought on the same week, then it is in no way
benefiting your seller. As they have already left feedback for you, they
will not be able to do the same now.
However, to make sure that repeat Feedback is calculated in the Feedback
Score, each transaction the member is leaving Feedback for must end in a
separate week. For Feedback, eBay defines a week as Monday through
Sunday, Pacific Time.

For example, a buyer wins an item on Tuesday, then receives it and gives
the seller positive Feedback on Thursday. If the buyer wins another item
that week with the same seller, the Feedback the buyer leaves will not
count toward the seller's Feedback Score. But if the buyer wins another
item the following Tuesday, the Feedback will count toward the seller's
Feedback Score.