Say I earn nothing, then a UBI of $100/month comes in; I have increased my income by an infinite percentage.
Say I earn $10,000 and the same UBI comes in, I have increased my income by 1%.
This results in a general flattening of the income v. population percentile curve, effectively flatting out income differentials and promoting equality.
Things do go haywire if the governing thought is "everyone should get enough money to buy an X" in some scenario where there are Y people and (Y-1) Xs floating around. It doesn't matter how much money people have if there are not enough goods.