Measuring the orientation sensitivity of a UHF RFID transponder, where do the deviations come from?
In all of the three graphs the read range is slightly better in 0 degree angle compared to 180 degree angle. In third graph the difference is most visible due to scaling o the graph. My first guess is that the tag antenna is not perfectly symmetrical and read range is a bit better in one orientation.

Also. If you decrease the power step from 0.5 dB to 0.1 dB that would make the graphs "cleaner".
Theoretically the orientation patter of a dipole antenna tag is a "donut". In your test you have the donut sliced in three directions. First 2 are similar, slight angle is due to misalignment. Third is the angle which shows donut as a circle.


You are also welcome to contact Voyantic for any questions - about the tagformance system, or RFID otherwise. 

Disclosure: I work for Voyantic.