Where does 63dB come from..?
The specs are at 50 ohms (and some give odd impedances, like, 1Ω/100Ω, which can be more representative of some situations -- or, at least, hints at what you might expect at very different impedances), and yeah, in a typical application you'll have something very different from that at most frequencies (SMPS being a typical example, where the source impedance is essentially the capacitance of the switching transformer, no resistance there).
If the pulses were IEC 61000-4-5 surge or similar, the frequency content of that is pretty low (a peak in the, uh.. 50kHz range?), and the amplitude quite high, so that you may expect the common mode choke to saturate and let pretty much everything through, slightly delayed. Or let it through regardless because the load side has a high common mode impedance (= higher cutoff frequency). This and other reasons are why the line-to-ground voltage rating is higher (2.5kV?) than line-to-line. You need to be able to ride through something like that.
Tim