As you’re discovering, video bitrates are hugely variable. A still scene requires almost no bits; a scene with fast action may need 10x the median.
This makes the “Average Bitrate” of a file very misleading.
Plex analyzes files to identify the moment-by-moment bitrate throughout the video. Plex also knows the buffer size of each client. It uses these together to calculate the streaming bandwidth requirements.
When you look at a file’s info in Plex, bitrate is the file average. But if you look at the XML, requiredBandwidths shows how much bandwidth is needed to accommodate bitrate bursts at different buffer sizes. Before a file is analyzed, a fudge factor multiplier is used.
Handbrake’s “Average Bitrate” mode controls the total file size, not moment-to-moment bitrate. A file produced with “Constant Quality” and “Average Bitrate” modes will have the same peaks and valleys in bitrate. “Average Bitrate” mode is how you create a file that’s the perfect total size for a CD or DVD, not how you target a streaming bandwidth.
If you are targeting specific bandwidth, and want to pre-encode your video, one option is to use Plex’s built-in Optimizer presets. They tightly control bandwidth & required buffer sizes to maximize video quality for specific bandwidth settings.
If you want to encode for specific streaming bandwidth yourself, there’s an exhaustive thread about it here:
