Decrees in bitrate at the begging of encoded video
 

Hi,
I have tried to encode a raw video with h264 using Gstreamer tool and also with h265 using ffmpeg tool, since I need this encoded video for real time transmission I choosed encoder properties to have just P-frame and I-frame,but I observed a significant bitrate decrease at the beginning of the video. I was wondering if anyone has the same experience or know the solution for this problem?

Thanks