Gpa.delta and F9P constellations

Hi,
After upgrading to pixhawk4 with F9P GNSS model, I had a lot of GPS glitches.
Trying to solve the problem, as few suggested, I tried to reduce the F9P’s constellations. But unexpectedly with the fewer constellations I got more GPA.delta spikes.

all constellations:

only GPS+Galileo:

why is it happening? and what can I do to avoid those GPA.delta values?