I do not think GW changes the alloy. They are most professionals, maybe a little company like warrior could make lots that hit the streets varying the alloy, but do not think GW do it.
In my humble opinion, that could be a recast. Figures are an alloy of Pb-Sn (lead-tin), sometimes called pewter. Tin are add tostrengh the alloy and is lighter than lead. Also lead is heavier and cheaper than tin, and also in liquid state takes more detail in the mould.
So it seems yoy had a recast. If your alloy is mainly from lead could be easier to bend, but is not likley to break when you return it to the initial position.
I speak from a theorical point of view more than a practical miniature-caster experience.
-- modify after reading Dave answer -- well if the metal surface etch when stripping the paint, and goes dark to all of you... then I think I was just thinking to hard for a regular phenomena... uuuupps
