One of my rats seems to be outcasted?

So I have three male rats. Ones older and that I got from a re-homing post. The other two are about 2 months old and came from the same litter. Or in other words, they came bonded.

But one of the babies pretty much always hangs out with the older lad, while the other baby is mostly alone. The two rats hang out in the hanging tunnel all day, while the lone rat sleeps on the ground.

They aren't aggressive towards the lone rat. They still play, groom eachother and are otherwise civil. But I'm just a bit confused why one of the babies are always on there own? Especially when I'd assume the siblings would stick together.