Usually, it's 5 minutes exactly. Here, there are 2 possibilities as to what could have happened:
Since you edited your post shortly after posting, someone could have upvoted within that initial 17 minute window, and decided to undo it later.
If someone with edit privileges really wants to undo their own locked vote, they can edit the post themselves, undo their vote, and then undo their voteedit. If this is done within the editing grace period, their edit disappears forever.