I do grooms cakes for rehearsal dinners all the time that I'm not making the wedding cake.
There are a few reason this happens from my experience. The biggest is this, the bride orders a cake, the date gets all booked up, then they want to order a grooms cake, but there isn't any more slots.
Another reason is not every wedding cake maker can make a crazy sculpted grooms cake. I get a lot of recommendations when someones bride wants a sculpted team mascot, or a car, or boat, or something else they don't really make...
And another is people just don't think about it like that. They have a list, and they go through and check things off, and in their brain, a wedding cake person is not the same as a groom's cake person.
AND the grooms family tends to "throw" the rehearsal dinner, where the brides family does the wedding. They use different contacts and recommendations.
I don't think it's at all strange to book 2 different people for each cake.