Is there a standard way of measuring? I would think that they'd get in trouble if there were true discrepancies, but one might measure entire interior volume, where the other might measure the volume of what you can actually put on the shelves. Maybe some are based on exterior measures. I don't know.

What has been discussed here frequently, is that the number isn't a highly meaningful comparison. 15 cu. ft. vs. 22 cu. ft. gives you an idea that one model is larger than the other. But what folks here have found is that a well laid out 20 cu. ft. fridge might functionally hold more stuff than a competing 22.5 cu. ft. poorly laid out fridge, where a lot of the volume is used in places where you can't actually store stuff.

In your example, they well could have the same amount of storage space, or the "smaller" one could have more. I would call them equivalently sized on a gross level, but the only way to judge the interior is to look. Your best bet is to go to showrooms and actually look at the models in question. Take some containers with you. Maybe fill a baggie with styro peanuts or socks or whatever you have in the house to be the size of a chicken, and another the size of a head of lettuce. That kind of thing. See how they fit.