The sequence number of the file does not reflect the shutter count. Period.
The file sequence number is nothing but a number added to name the files. Your manual will describe how you can influence it. There is a setting of 'pick up sequence number from existing file' (default!) and 'start from zero'. Probably some sales man popped in a card with existing files and the camera picked it up, keeping the sequence.
Battery counter is a different topic altogether. So please, before suspecting any cheat, chop and whatnot spend some time reading the manual from beginning till the end and back. There are tools that can extract the shutter count from the image file. Not sure yet whether they are up to date for 7D.