I have also had nearly perfect agreement between the AquaCheck salt test strips and the Taylor drop test (tricky to do but very good when used correctly) across a number of bottles of salt test strips There have been occasional reports of AquaChek salt strips reading way off, but most people have very good results.
It is extremely common for the digital testers built into SWGs to be way off.
Electronic salt testers can drift substantially over time. If you are using an electronic salt tester make sure that it has been calibrated recently.
Most refractometers are not very precise in the range used by swimming pools. They are normally optimized to measure salt levels closer to ocean water, ten times as much salt as you use in a swimming pool. There are some better ones out there that can precisely measure lower salt levels, but they tend to be expensive.
In this particular case, it really doesn't matter. Both the pool and the SWG will be fine with salt anywhere from 3,000 to 4,500. As long as the SWG is happy with the salt level and it is producing chlorine, then leave things alone and don't worry about it.
The SWG is measuring the resistance of the water to electricity, which normally corresponds to the salt level fairly closely. However, there are a couple of other factors that can affect the resistance, which can cause the reading to differ from the salt level. What really matters to the SWG is the resistance, not the true salt level. So, if the SWG resistance based measurement comes out in a range the SWG is happy with, the true salt level really doesn't matter.
The situation is a bit more complex when the SWG is measuring an invalid salt level, the salt level might be wrong, or the sensor could be broken. But you don't need to worry about that case, since the SWG is reading a good salt level.