The bluestones appear to have been, at least in part, pulled from older structures which were dismantled as their second use as quarries. Since one of the large inner Stonehenge structures has a lintel that bears the marks of a correction during carving, it's not impossible that it had previously been part of another structure as well. :^)
Oh, I know of that mistake. I had never considered it to be a lintel from an older structure. Wow. That’s a thought!