Why is 224 bit ecdsa faster than 192 bit ecdsa?
Both curves have similar form and primes close to powers of two ($2^{192}-2^{64}-1$ and $2^{224} - 2^{96} + 1$), so you wouldn't expect large differences in performance – all things equal, P-224 might be anywhere from 30%  to 60% slower due to the computational scaling of curve operations.
However, in practice different implementations will have different performance and some curves' will be better optimized. In this case it looks like P-192 might only have a 32-bit portable implementation and P-224 an optimized 64-bit one:

Changes between 1.0.0h and 1.0.1  [14 Mar 2012]
[...]  Add optional 64-bit optimized implementations of elliptic curves NIST-P224, NIST-P256, NIST-P521, with constant-time single point multiplication on typical inputs. [...]

(Note: I haven't actually checked if P-192 is unoptimized. You should do it with your version if you are interested.)
As for why they would optimize P-224 better, it is the minimal acceptable size according to e.g. NIST recommendations. Doesn't make much sense to spend time optimizing something that's recommended not to be used.