What determines the chosen bitrate with ffmpeg constrained encoding example?
think of it this way, your driving a car, and you have a target speed of 25, and a max speed limit of 25. Then you stop at a red light for 30 seconds and your average speed drops to 20. It is now impossible to reach an average of 25 again without going over the max. you would need to drive at the max speed an infinite amount of time to get the average back to 25. 

When encoding the video, the encoder reached a frame that took less bits to encode than average (low spacial or temporal information.  For example two identical frames), so bitrate temporarily dipped, and now it can never reach max. 

What you need is a way to say, if we drop below average, we are able to go over max for a small period of time to bring the average back up. This is called “vbv” and you should use that if you want to target a bitrate.