It is possible that some of the internal memory is being consumed by POI's (Points of Interest) files and or SPEED CAMERA files.
TTActivator allows you to remove SPEED CAMERA files, but to date, does not deal with POI files.

You can try and remove the SPEED CAMERA files with TTActivator and look at the reported memory size to see if that is the cause.
Access to the POI files is via MyDrive Connect which will allow 'adding' and 'removing'