As gembo says, there are quite a number of factors. Not least I guess as well is what we see as muppetry. For me it's someone passing too closely in a way that makes me 'react', or a full on SMIDSY, or the classic left hook. I don't necessarily count things like 35mph in a 30 zone etc. It's wrong, but not really 'muppetry' (in my eyes, but obviously not in others).
So for instance I was out at 8 this morning on a fishmonger and butcher run. 9 miles, primarily on roads, some of them busy, some of them small and narrow, some one way, some with bus lanes. I think I was probably passed by around 60 cars in total. I had one person pass rather too closely just a few minutes out of the door, then every single other driver was absolutely fine, so 1 in 60.
On Monday I did a 40 mile loop out to South Queensferry, and back via the Gyle and the canal. About half the distance, maybe a little more, was on the roads, and I had not one single moment of muppetry. Not one. And must have had a 100 cars 'at least' that went by me.
However on some commutes, just 4 miles, with a mile of that off road anyway, the level could be as high as 1 in 20, which could be put down to a number of factors, but I'd assume the biggest factor being volume of traffic causing much frystration (not an excuse, just saying that could be a reason for a higher count).
I have to admit that as with gembo, if my ratio was an 'average' of 1 in 15 I think I'd be giving up cycling! Considering on a busy road 15 cars can go by every two minutes, on a half hour ride that'd be 15 times your life is put in genuine jeopardy!
What would be a very very interesting study would be for a whole commute and leisure rude to be filmed, and have people separately noting the moments they thought they'd seen some muppetry, and then compare the counts, as I suspect there would be wildly differing figures, simply because we all view things differently.
I do think, though, that we do sometimes forget most of the good driving around us, focus on the bad, then see that as a disproportionately high amount (a bit like drivers seeing 7 cyclists waiting diligently at red lights, then one going through, and that somehow being inflated into 'every' cyclist runs red lights - I'm sure the psychology behind it all is incredibly interesting, but that's most certainly not my field, so I doubt I could even come up with a vaguely sensible reasoning!).
So in short, bax, fancy being the guinea pig filming your commute?