There can be many reasons for this. Note 549064 should be reviewed for potential reacons.
I would suggest that comparing V_V2 & VA02 is generally not ideal. They are essentially different transactions that so different things, so it is impossible to expect same results. V_V2 selects a range of documents, unconfirms these documents, then re-allocates the available quantity. VA02 will only check one document, no other documents get unconfirmed. This means you could get different results.
Perhaps if you provide screenshots, it might be clearer what is happening for you.