GPS accuracy is based on a number of factors not the least of which is the refraction of the radio propogation through the ionosphere.
Fortunately, the corrections supplied by the various services (WAAS and the like) do a pretty good job of taking out the effects partly because it is assumed that the ionosphere is largely homogenic and what affects the receiver will also be similar at the base stations that provide the reference signals for the correction.
Fortunately, the whole GNS system has had some remarkably quiet ionospheric conditions over the last couple of years to optimise the models and refine the positions of the Ground station network on which the whole thing depends.
When and if the sunspot activity increases again, the disturbance of the ionosphere will degrade the accuracy of the positioning calcs and you may well see some "outliers" that exceed the limits that the equipment specification provides.
Therefore, be aware that the current accuracy that we now enjoy, may not necessarily be provided in the future.