Honestly, I don't think I've seen a rifle that the handguards didn't move some. It's more prominent in the CAR handguards, from what I've seen. Are your BM's CAR's? If you got two Bushmasters that don't, I'd say you got lucky.
As a point of interest, I get almost all my handguard components (delta parts, caps, even handguards) from the same place as Bushmaster gets theirs, so there's not really anything there that could be different between theirs and ours.