That's how I believe most people think.
1GB is possibly not enough, 2GB is probably enough, but I might one day need 4GB. Is the extra $10 worth it to have 4GB in case I might find that beneficial in future ?
That extra 2GB might never get used but it's insurance against wishing one had got that on day one.
And we are all familiar with software growing and needing more of everything as time goes by. 2GB might do me for now, but what when there's a 64-bit Raspbian, a 64-bit Chromium or Firefox, Scratch 4 ? Will I want to be doing something I never thought I would be doing ?
I don't know and there isn't an answer; 'play safe' and get the largest there is against the risk of having not done that. If only the Pi Store sold crystal balls.
There are always a mix of purchasers driven by different desires and constraints, those who want biggest and best, some even wondering if they should get 4GB now or hold out in the hope of 8GB, those who want it for what they do now, those looking at a longer term investment, optimists and pessimists, and those limited by what they can afford.
I can understand why it was predicted the 2GB was going to be the most popular, but I can also see why 4GB is turning out more popular than expected.
Maybe more market research would have made it clearer which would be most popular but it doesn't seem a particularly difficult problem to resolve - Just build more of what's wanted in the next batch.