The nonce is taken modulo the order of the prime-order subgroup. For DSA that's generally a 256ish-bit prime (e.g.: choose a 1024-bit prime p such that a 256-bit prime q divides p-1; then there exists an order-q subgroup of Zp).

For P-521, the base field is 2^521 - 1, but the modulus used when computing the nonce is not that value, it's the order of the P-521 curve. By Hasse's theorem, that's roughly p +- sqrt(p), which is essentially p for such large numbers (the cofactor of P-521 is 1, so the order of the group is prime).

So: both are 521-bit numbers, but the group order is less than 2^521-1. Its hex representation is 0x01fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffa51868783bf2f966b7fcc0148f709a5d03bb5c9b8899c47aebb6fb71e91386409.

Ahh. Thanks! (I don't think about FFDLP much, as you can see).