Suppose your agreement says your agent is owed three percent, and the seller ends up offering two. Somebody absorbs that gap, and the clause that decides who is usually a single sentence inside the compensation paragraph: "Buyer shall pay any deficiency", or "the difference shall be due from Buyer at closing", or "Buyer remains obligated for the balance."
What makes it expensive is not the wording but the timing. The gap comes due at closing, in cash, on top of your down payment and closing costs, at the exact moment your liquidity is thinnest — and it is not financeable into the loan.
The shapes this clause comes in
Some agreements make the buyer liable for the full difference with no ceiling. Some cap the exposure at a stated dollar amount or percentage. Some limit the obligation to whatever the listing side actually offers, so the gap is never the buyer's. And some are silent, which is its own answer that a lawyer in your state should interpret, not a website.
Whichever shape yours takes, it interacts with the concession your agent negotiates for you in the offer — the person writing that request is the person the shortfall would otherwise fall to or from. That is not an accusation of anyone; it is a reason the number belongs in the open between you.