I contacted the Ft. Delaware Society, which was able to distinguish between the two Williams, and had the same observation. The gentleman there who e-mailed me thought perhaps he was home on furlough when captured, and the amputation was in error. It doesn't make sense that they would bother to capture a disabled soldier and send him to a POW camp.
I want to go down to Ft. Delaware and look at their camp records. It's only about 2 hours from me. The other interesting thing is that he would have been about 40 with 6 children when he enlisted. I think it would have been WV that issued a pension to his wife. Looking for it is my next step.