why is the -S or --split-filesize Split so far off ?

i am using this command - ```python3 ffmpeg-split.py -f 'somevideo.mkv' -S 4194304000``` whereby i intend to split the video under 4000 MB ( = 4194304000 bytes ) but the resulting output creates splits with filesizes like 3200-3300 MB . i know its meant to be approximate but 20-25% variation feels bit too much . Is there any way to make it split closer to the -S filesize passed ?