The one that have the barometer in it use the combo or a 3D sat lock and the barometer to get your altitude. A 3D sat lock is actually more accurate but less possible...After time it does get better as Torgrot mentioned.

I do not believe the 205 has a barometer in it. I think it relies on just the 3D lock...either way when you upload it I believe it smooths it out pretty well....