pnoise jitter and pnoise time average discrepency: what is the problem?

Hi everyone,
I try to get the jitter from a clock generator, but I also need the phase noise curve for other reasons:
- When I simulate my circuit with the pnoise/jitter method, I get a RMS Jitter in a given bandwidth of 594.386fs (Jee>RMS).
- And when I simulate with the pnoise/time average method, by calculating my Jitter from the Phase Noise curve, I get 269.2fs.

Assuming that the pnoise>PM>Phase Noise plot is the IEEE definition, say Single-Sideband-to-Carrier Ratio, so-called L(df), "L-script of df",
I compute the RMS jitter as follows in my calculator:
[1 / sqrt(2)*pi*f0] * sqrt{ INTEGRAL[ L(df).df ] }

Of course, here L(df) is converted in linear, L(df)=10^LdBc(df)/10

In your opinion what do I miss??

Thanks a lot in advance for your help!