It probably lost its leg previously without you noticing it. The thinner and whiter leg you see after molt must be the new one. It'll be just like the others in one or two more molts.
My daughter refers to them as arms. Considering she's actually arachnophobic any interest is good interest. She now claims myIts easy to mistake their pedipalps as a leg. I've had folks say, they have 10 legs??? i say nope. the front pair is their padipalps they feel around with.
It's easy not to notice