You know, I can't make sense out of it, but Annabelle always does the same thing. When I take her to the dog park she always ends up ignoring all other dogs but the GSD's.
My beagle tends to hang out with the "bigger dogs". He won't play with the small dogs at the dog park. He has a Mastiff friend, a Doberman friend, and he ran around with a 3 pack of wolf-hybrids last time we were there. He wouldn't leave them! It really looked funny!