Fenix 6 Sapphire gps not accurate

Selective availability has been gone since May 2nd, 2001. That was responsible for intended inaccuracies of a hundred meters or so, for civilian users.
Still, it's true the L1 C/A signal is less precise than the signals used for military users. You can not expect better accuracy than 3 meters. Note that this is CEP, Circular Error Probable. In other words, at the very best and most favorable conditions, it's something like 95% probable that you are within a circle with a diameter of six meters from where the receiver tells you that you are.

Now, this is under the best conditions.

• Satellite geometry should be perfect, with an even distribution of satellites over the sky.
• There must be no obstructions, like buildings, trees or other things.
• No excessive electrical charges or other foul play in the ionosphere.
• You must have waited long enough to be sure you have up to date ephemeris data from all satellites used.
• You are not mvoing, and haven't been for some minutes.

In the real world, you have probably violated most of the above conditions. Suddenly your expected accuracy has grown to perhaps 10-15 meters. So suddenly are are in a circle with a radius of perhaps 30 meters, and you still aren't 100% sure you are inside it. In these cases it comes down to the designers. How should they handle the constant changes in reported position? If you cross the road, then go back, you want to see that. But if the foul signal says you crossed the road, then went back, you don't want to see that. What errors to mask, and which not to mask, that's the question.