So. What are theories?

Does this mean the bluestones were used somewhere else before being reused at Stonehenge?

Bluestone was used elsewhere as well as at Stonehenge?

The bluestones for Stonehenge were brought from Wales and worked at these places before moving them to Stonehenge?

The bluestones were in fact eratics found at these locations and were worked there before being moved to Stonehenge?

The latter sounds quite likely to me.