Why is the fps not constant as configured?

Hi all,

I jiust wondered why my configured Stream with 25 FPS is not constant while streaming.

My setup is: SDI-Video in with PAL SD (720x576), Output is e.g. 640x360 with 1.600kBits in h.264 with 25 fps and AAC Stereo Audio. I am using a 8 core machine with Osprey SDI-capture card and running at about 20% CPU capacity while encoding. I use the "auto adjust" feature with drop frames option and a max. buffer size of 1 second.

When I start encoding the average fps is at abou 24.78 and during encoding it levels up to 24.94 fps - so this is nearly 25, but why is it not constant and what can I do to reach my configured 25 fps?

The quality is good and it is not because of the missing 0.01 fps - but I want to understand what happening - Could somebody explain that to me?

Thanks, Marco