If I was buying Ritz crackers for $4 before, maybe now they're $5. I'm making more money, and it's just $1, so I might not even be paying enough attention to notice, but if everything goes up by a similar amount, then I'm spending significantly more on the same items than I was before, and might end up dropping $100 of my new UBI money on groceries without even making a change in my shopping habits.
Now, a lower income person might be buying store brand crackers that only cost $2, but now they're $3, so the same situation occurs.
These are hypothetical numbers of course, but I wouldn't be surprised if a situation like that occurred, given that every company would know exactly how much more money is now in everyone's pockets. Every product goes up just a bit, just to take a bit of that UBI pie.