@poVoq I agree with you AGPS is here to improve the Time To First Fix (TTFF) but it has no influence on the positioning accuracy.
We would just need to find out what output of the Nexus 5 GNSS receiver is used in UT; most of the time a receiver offers different types of outputs, more or less accurate: GPS alone, GPS coupled with GLONASS (seems like the Nexus 5 GNSS receiver uses both constellations), GPS coupled with SBAS (WAAS over the US, EGNOS over Europe, etc.).
Maybe we should get in touch with the developers of UT regarding this specific issue in order to find a way to improve the positioning accuracy. How can we reach them on that subject?