To continue the observations from John DF4UE (see thread on 2 Hz difference).

Using wd-query on the Kiwi that has a RFZero supplied 66.66 MHz reference and GPS; local but over the air received KiWiSDR decoded signals (John used coax from Tx to Rx).

I see as lowest local spread 4-10 mHz for all transmitters I used. When I average over 4-10 mHz these are the figures per transmitter

Yaesu FT817nd as Tx : 5.4 mHz. Nothing but the standard box, no GPS, no reference signal.
Using RFZero as Tx : 6.0 mHz, GPS aided, little piece of foam on the oscillator, piece of foam over the otherwise open pcb
Using Hermes Lite V2 as Tx : 6.6 mHz, no GPS, no reference clock signal, nothing but the standard box

Other observations : quite frequently an external reception has a lower spectral spread than the local reception. Seen with OE9GHV, G3ZIL and EA8/DF4UE
Highest local spread can be over 900 mHz, same for external locations

Spectral spread of 6 mHz almost never happens, only 4 times out of 12.000 observations