You would think they would have a method of screening out handstamp re-entries from entries. (I don't know that they do, but from a marketing perspective, counting re-entries could tell you if people are leaving for lunch, dinner, etc.)
Now, as for one person visiting the same park on multiple days like you, you are right that attendance doesn't tell the whole story since you don't know if people are coming in for a little while or all day. I often wonder about the Epcot numbers in years past b/c I know we'd be there every other night for dinner when we stayed at BC.