Why the period between two captured frame is different?

Hi, I use Decklind Quad card to capture video. And i use the SDK sample InputLoopThrough, and add some code to record the time when VideoInputFrameArrived() is called. We get the following log files.

// configuration: 720p50.

The time period for most of the frames are correct (22 ms per frame).
But every 4 frames passed, the time period for a frame is only 11 ms. It,s very strange. Could any one give an explaination about this. I also try other configurations, most of them have the same problem. Thanks in advance.

Notes: i use windows timer to record the time.



001 time elasped since VideoInputFrameArrived() called: 5828296.108576 ms frameDuration = 12.
002 time elasped since VideoInputFrameArrived() called: 22.251310 ms frameDuration = 12.
003 time elasped since VideoInputFrameArrived() called: 22.251756 ms frameDuration = 12.
004 time elasped since VideoInputFrameArrived() called: 22.269161 ms frameDuration = 12.
005 time elasped since VideoInputFrameArrived() called: 22.160714 ms frameDuration = 12.
006 time elasped since VideoInputFrameArrived() called: 11.093746 ms frameDuration = 12.
007 time elasped since VideoInputFrameArrived() called: 22.205789 ms frameDuration = 12.
008 time elasped since VideoInputFrameArrived() called: 22.192847 ms frameDuration = 12.
009 time elasped since VideoInputFrameArrived() called: 22.206682 ms frameDuration = 12.
010 time elasped since VideoInputFrameArrived() called: 21.975952 ms frameDuration = 12.
011 time elasped since VideoInputFrameArrived() called: 11.065630 ms frameDuration = 12.
012 time elasped since VideoInputFrameArrived() called: 22.269608 ms frameDuration = 12.
013 time elasped since VideoInputFrameArrived() called: 22.198202 ms frameDuration = 12.
014 time elasped since VideoInputFrameArrived() called: 22.124119 ms frameDuration = 12.
015 time elasped since VideoInputFrameArrived() called: 22.223640 ms frameDuration = 12.
016 time elasped since VideoInputFrameArrived() called: 11.070093 ms frameDuration = 12.
017 time elasped since VideoInputFrameArrived() called: 22.225426 ms frameDuration = 12.
018 time elasped since VideoInputFrameArrived() called: 22.170979 ms frameDuration = 12.
019 time elasped since VideoInputFrameArrived() called: 22.229888 ms frameDuration = 12.
020 time elasped since VideoInputFrameArrived() called: 22.236136 ms frameDuration = 12.
021 time elasped since VideoInputFrameArrived() called: 11.059828 ms frameDuration = 12.
022 time elasped since VideoInputFrameArrived() called: 22.237922 ms frameDuration = 12.
023 time elasped since VideoInputFrameArrived() called: 22.275409 ms frameDuration = 12.
024 time elasped since VideoInputFrameArrived() called: 22.151789 ms frameDuration = 12.
025 time elasped since VideoInputFrameArrived() called: 22.169640 ms frameDuration = 12.
026 time elasped since VideoInputFrameArrived() called: 11.164259 ms frameDuration = 12.

720p50 has a frame duration of 20ms.. so the average is right.