Thanks for all your comments. I didn't think about different schds for the pipe. It make sense that they probably want to make one size gasket to cover different pipe schds., as long as the compression area is sufficient for the pressure rating.
I'm using class 150 1 1/2" pipe raised face flange. It has an ID of 1.37" I was looking at the gasket from MMC. For 1 1/2", the gasket ID is 1 29/32". I understand the gasket OD should be between the raised face OD and bolts, but I don't understand why the gasket ID can't be a little bit...