There are filming tricks that can make people look different heights than they are. It's also common to use such tricks to make sure that your female lead doesn't look taller than your male lead, because that's seen as ~undesirable. I wouldn't be surprised if they did something to make sure that Karen didn't look taller than Matt.

Of course, they could achieve the same thing by just not having Karen wear high heels, but US tv is pretty obsessed with putting all women in high heels.