So,oOption A is that you "donate" more goods into the system than the UBI you are consuming, which is all fine and dandy for the rest of us. Option B is that you are consuming more value that you are donating, which is less fun for the rest.
So, some people will need to overproduce in order for the people who under-produce to get their UBI worth of goods. Mind you, this isn't different than the current state of affairs. The big question for me is how would the introduction of UBI ultimately affect that ratio, given that you would be less disincentivized to stay with the underproducing group?