I've set up SPF, DKIM, and now DMARC (reporting only) on my site. Sometimes when my site sends me an email, I can see that my GMail inbox has evaluated SPF:Pass, DKIM:Pass, and DMARC:Pass, but sometimes I only get SPF:Pass and DKIM:Pass, with no mention of DMARC. Do mail servers sometimes skip DMARC tests? Maybe for whitelisted domains or cached senders?
Generally speaking, DMARC is only needed when SPF or DKIM lookups fail, as their main purpose is to say what to do in the event of a failure. There's no particular reason to look up DMARC if SPF and DKIM pass, but I guess gmail must just do it anyway sometimes. Many mail servers don't do SPF, DKIM, or DMARC tests at all. If you had a case where gmail was failing a check and not looking up DMARC, I'd be a bit more concerned.