It might be a lender restriction, could also be that the builder is reducing the commission because they increased the credit.
But, this should have been clearly explained to you, so if that's the reason not sure why the agent wouldn't just tell you.
If it's not a lender restriction, and not a commission reduction by the builder, then IMO, the agent should stick w/what was promised and you should hold them to it.
Hopefully you got it in writing. The agent commission and any rebate should be clearly spelled out in the "Exclusive Right to Represent Buyer" form. Should have been signed before/while you were shopping for houses, if not, it may have been included in the larger contract package. Look through your paperwork and see if you can find it.