start time command

Hello, i have a problem with the --start-time command line.

In fact if i play a file with this command line : "C:\Program Files (x86)\VideoLAN\VLC\vlc.exe" --start-time=1800 --stop-time=2000 "\sa-cluster\plasma\plasma.mpg" vlc://quit

My file start at 1178 seconds and not à 1800 seconds : 35% of difference. If i try on another file with another bit rate it's for example --start-time 3600 and real start at 2917 : 19% of difference.

It's normal? or i do a bad use of --start-time?

Thanks a lot for yours answers.