I found that a bit strange too, their pack instinct is as strong as any of their other instincts, unless those particular dogs were not good with other dogs in general, I've known the odd snappy one, but even so, they generally tend to warm eventually and fit in quite easily.
Oddly, when we went to see our rescue Beagle, we had to take the Beagle with us (which was a nightmare in itself as he could never keep still, and it was a 120 mile round trip) to see how they got on, they ran around like idiots playing with each other, had a fight, got told off, stopped, then carried on running around like idiots playing with each other, after that they got on great pretty much their whole lives, the only issue we ever had was food, the rescue always trying to steal the others, but in my experience, any dog that's ever known what it's like to starve, that never really leaves them.