It's not 'both internal GPS and AGPS'. Its A-GPS, period. The N900 comes with a (poor) TI GPS chipset, which needs assistance if you want a reasonably fast lock. In other words, it works better if you're online at the time.

Sygic gets its GPS data through the GPS subsystem on the N900 (which I guess is a version of gpsd, the gps daemon). Thus you could also switch to an external Bluetooth GPS by changing the setting in the control panel. That comes with its own set of problems though, basically the software on the N900 does a poor job with the NMEA data so that not all the info comes through all the time, and that combined with Sygic wanting all the data (not just position, speed and altitude) all the time can be problematic.