Was this a refurbished gps. If you allow the gps to lock on to the navsats does the number change? One possiblity is that the number is altitude in meters. If the unit was refurbished the orginal user may have installed tha third party plugin called height. you can check this by looking in the gps's drive with windows explorer for the following files and folders.
1 folder - "height"
2. folder -"ttninit"
3. file "ttn"
If these files / folder exist you can simply delete from the gps and then perform a reset.
********************************************************************
Report map errors at http://www.tomtom.com/mapshare/tools/
********************************************************************
Backup your gps --- for details see
http://uk.support.tomtom.com/app/answers/minor_detail/a_id/13354/
or
http://www.tomtomforums.com/general-tomtom-discussion/17864-how-backup-contents-your-unit-computer.html
********************************************************************
