It doesn't make sense, unless somebody (the miner who found the block) wanted to burn the #sats and recreate new ones - all paid fees are getting burned and the miner creates new ones. Sorry, can't find any link explaining it, RN.

As for your question: a Virtual byte. https://bitcoin.stackexchange.com/questions/89385/is-there-a-difference-between-bytes-and-virtual-bytes-vbytes