I would disagree that 20% is the norm in the USA. This may be common in a few places such as NYC (OP didn't mention where the meal occurred) or when the person paying (usually with OPM) thinks that this impresses others and there might be tipping "suggestions" (either travel/etiquette advice or those automatic calculators), but it's far from universal.

IMO in most places, the benchmark would be 15% of the bill BEFORE TAXES for adequate service in most full service restaurants. I'd tip a bit more for better service, but not go to 20% unless it had been very cordial and professional outstanding service in a more formal or luxurious restaurant.

I suspect the OP reported on the check including tax, so the tip might actually have been 15% or higher.

I'm also surprised that restaurant policy didn't just add whatever they consider to be the standard tip (often kindly rounded up to make it worse, or even miscalculated in their favor) for a party of this size, of course hoping that customers don't notice and will actually tip twice.