That could just be in your GPS map database, since speed limits don't change very often. One presumes the map database is updated from time to time.

Of course, the databases aren't that good yet. In fact, they don't have a far more valuable piece of information -- what's the real typical speed on that street at various times of day, something they could easily learn from data logs from taxis, buses and willing car drivers, allowing them to calculate true "quickest route" paths -- even combined with light schedules -- rather than the often sucky ones they have now.