Ok, I admit one of the GPS units is over a decade old, but the other is only a few years old. Phones, in particular, use added inputs like local WiFi networks, and accelerometers to further refine the estimated location that aren't available on a dog collar.
I think that there are extenuating circumstances for the GPS, in terms of how they do the math to arrive at a fix, and some of the math approximations that are in use.
However, my basic point is that under ideal conditions, consumer grade GPS units are +/- 3m (9ft) horizontally, which is tough on a dog in a yard, especially when those limits wander with differing satellite constellations and geometric fixes. (Imagine the poor dog; "WTH? I could get to the rose bush this morning...")
The invisible fence solutions have the advantage for the dog and dog training, that the boundary doesn't wander. (Not pushing that solution, just looking at it from the dog's perspective.)
All the best,
Peter