How does ArduPilot calculate HDOP?

Hi, I am having trouble understanding something. I am using Cube Orange and the HERE3 GNSS receiver. The GPS HDOP reported by ArduPilot is around 0.7 constantly for me. I am having trouble believing that the GPS HDOP is really 0.7m, since every article on the web says that GPS HDOP can go down to 3m. Even the HERE3 docs say that the best horizontal accuracy is 2.5m. Also, when using RTK, the number stays at 0.7 All of this makes me wonder what is behind that number. Does it include other sensors and can provide better than 2.5m accuracy with it? Is it actually HDOP or some other way of calculating “accuracy”?
I am lost, thanks for any help in advance!

HDOP is not accuracy in meters.

Accuracy is reported in the acc measurement.

Please read some more internet posts to understand what HDOP is. And it is calculated by the GPS receiver, not ArduCopter.

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I see, thanks for your reply! I had it confused with accuracy, thanks for the clarification.