gps deviation for a couple of seconds. Why \ How it is possible?

GPS works with weak radio signals coming from satellites in 20000 km altitude. Many electromagnetic signals can disrupt the reception, sometimes by accident and sometimes planned:

www.incibe-cert.es/.../spoofing-and-jamming-over-gnss

thanks a lot, but i think that garmin should "clean" the track from these errors. It could be done with a very simple algorithm... 

Corrected CPE for EPO. Different filename, same purpose - to provide satellite ephemeris data to speed up signal acquisition and assist in activity tracking,

It’s possible for the reasons stated but that looks a little outside the norm. Force an update of the EPO file by syncing with GARMIN Express. Leave your watch outside with a clear view of the sky and a GPS activity selected for a 10-15 minut4s to get a GPS soak. Then try again.

If it happens again, consider contacting Garmin Support.