people with iGPUs are far far less likely to be using Steam. They're a lot more likely to be playing Zynga games or some Chrome game.
Steam currently has 275 FREE games, of those the vast majority can be played with HD4000 and above iGPUs at 720p or above.
You can play the vast majority of them on a AMD A8-A10 APU at 1080p(especially with Kaveri APUs).
For example, the systems in my sig - exactly one of them has Steam installed. Can you guess which one?
And these are all dGPU boxes - none of them except the desktop capable of playing games decently (because they're old). That being the case, I don't stick Steam on them to pointlessly consume resources. The logic follows that many, many iGPU users do not play dedicated games like those on Steam, and hence are under-reflected.
At home I have a main Gaming PC with an i7 3770K and HD7950 with steam installed. I have an SFF HTPC with an AMD APU that also has steam installed to play games on my large TV. Car simulators and other less demanding free games that my wife and children of friends coming home play. Wife using a laptop with steam installed as well, fully capable of playing those free Steam games.
My main Gaming PC has took part in the Steam Hardware Survey ONLY two or three times the last 24 months or so. The HTPC was second with 3 or 4 times if i remember correctly and wife's Laptop took more than 5 (she plays more hours than me by far being home with the kid all day).
But even within your (bizarre) use case of only using one system per month, all of your machines at least get counted. Multiply your use case by 10 people doing the same thing and you'll find that statistically the ratio of dGPU:iGPU is dead spot on.
See above, my main PC hasn't participated on Steam Hardware Survey the last 6 months or more. There is no data from my main Gaming PC in the current Steam Hardware Survey.
Ohh one more thing,
NOT everyone with a steam account is taking the survey every month. That can happen because you might not open Steam that day the survey it takes place.
I really dont remember when was the last time i took the survey.
So Steam accounts may be 125M, but not everyone takes the survey every month and that also skews the results.