How are polars made? Are they derived from actual conditions or calculations? If they come from actual tests, how can you have the same sea state at wind speeds from low to high?
They are calculated. The Velocity Prediction Program that is used for ocean racing handicaps (used by US Sailing) includes calculated sea state for each true wind speed. The model gets better all the time and the results are a better reflection of true performance than they used to be.
Observed polars for a boat are totally dependent on the quality and calibration of the instruments. Boat speed is a tough one. If you spend time with instrument calibration you will find that most boats do not have a linear speed calibration. If the knot meter is calibrated for 3 knots it will probably have an error at 6 knots. GPS speed is completely irrelevant for creating polars or for knot meter calibration.
Wind speed and angle measured at the masthead must be adjusted for height over the water and for leeway angle and for the upwash from the sails.
Getting all this right is very time consuming and most systems have no way to calibrate for more than one boat speed or wind speed much less for different sailing angles and sail combinations.
In other words, don't expect to hit the polars from a VPP all the time. Use the best angles as guidelines after you have your instruments calibrated as closely as you can get them. Then make notes as to best observed performance at different speeds and angle to use as "personal best" targets to beat. If you are constantly faster than the polars ... your knotmeter is not calibrated correctly.
Randy