Basically, only the character change count of your last edit (within the 5-minute boundary) is counted. This gives odd results: if — after a major update — you change one character to fix an SO markdown bug or a spelling error, the count will be one.
Probably not high on your list, but I'd like to suggest to show the count of the combined changes, which will raise less eyebrows I reckon :)
Example: see revision two here, where the big green block really isn't one character long.
EDIT: In case this revision tree ever changes or this bug ever gets fixed, here's screenshot:
PS: this issue seems solved for new edits, see answer below, can someone close this?