Ok, so here's what happened:
I had 3x ~250GB disks in a pool. One was dying, so I removed it. This left me with the 3.05GB of unusable for duplication.
I replaced that with a 480GB disk. It then read there were 17GB of "unusable for duplication."
So, freaking out, I did a balancing pass, and it went back down to about 3GB afterwards. I suppose there's some math that might leave 3GB of unduplicatable space, but given the slight variances in disk capacities (~240GB/~250GB/~480GB), I'll accept it.
No data lost and everything else appears to be in order. Thanks again, Drashna!