Problems at the satellite
Even though the satellites are very sophisticated they do account for some tiny
errors in the system.
The atomic clocks they use are very, very precise but they're not perfect. Minute
discrepancies can occur, and these translate into travel time measurement errors.
And even though the satellites positions are constantly monitored, they can't be
watched every second. So slight position or "ephemeris"
errors can sneak in between monitoring times.