Comment by pbsd

The first version of the X9.62-1998 standard, where these curves first showed up, only had P-192, P-256, and the prime192v{1,2,3} and prime239v{1,2,3} curves. These also happen to be the curves with the earliest timestamps (along with P-384 and P-521). It also contained 163-bit curves, but none of those correspond to the later B-163 curve.

So yeah, it seems plausible P-224 and the B-* curves were generated at a later date.