[atlas] Question about rt definition of DNS measurements

Hi! When doing DNS measurements I see that the average "rt" time is considerable higher when setting the DO bit. When I check my name servers manually with dig (with and without DO) I do not see any considerable differences. Thus, I wonder how "rt" is measured in the Atlas probes and if the probe does some DNS/DNSSEC relevant processing during the "rt" interval, or is it really just the time between sending and receiving of the packets on IP layer? Thanks Klaus