A common question maybe need explaination of DirectShow working structure?

[Valeron]

For instance,I open some video file in directshow with MPC or Media Player 6.4 as well.Then view the real time frame rate via video renderer's information,but no matter any type of video I playback(include XviD encode AVIs,WMV9 in WMV container and RV9/10 in RMVB,even with high bit rate),the actual frame rate of certain clips is always far lower than the expected ones at the very first seconds of that video,23.976 for the IVTC content and 29.97 for none IVTC content.
But the playback is smoothly(at least I never feel frame drops with my eyes).And then the frame rate will go up little by little.After a certain period,usually need at least one minute or more,it certainly can reach the expected rate.Though not exactly the same.
Anyone can explain in theory??

[theReal]

I guess it shows the average frame rate over a few seconds - so in the first few seconds the average is still lower than the actual displayed frame rate.

[Valeron]

Originally posted by theReal
I guess it shows the average frame rate over a few seconds - so in the first few seconds the average is still lower than the actual displayed frame rate.

But how did the video renderer caculate "average frame rate"?
Isn't it follow the simple formula:total decoded frames from frame 0 till the very moment it show the rate/playback time from start?
So why the actual average rate is so low?
Isn't that the rate caculated by seconds when encoding but by the whole clips?
Or the encoder will tweak the rate for different scenarios?
Feel so confuse.
All clips I refer to are 2 pass encoded clips.