Is cycling in my DNA?
If you really want to find out, look for a local time trial route and see what people can do on that.
There's also an advantage coming up behind someone - not so much the aero advantage as you have to get close for that, but the knowledge that there's someone to catch up with - your head goes into race mode, even just a little, but theirs doesn't.
Some riders have all the gear but no fitness. Others are having off days. I've seen the effects you note from both sides. Within the last few days I've blasted past 2 roadies uphill while on my shopping bike with a big backpack, and struggled to make much headway at all on my faster but still heavy bike after wearing myself out earlier in the ride (140km with some rough gravel). Flat out on the TT course near me I might average 21mph for half an hour, commuting on the same bike little more than half that unless I'm rushing to get somewhere. But my fast pace would be almost last looking at events on that course; winners are 50% faster than me.