In my experience you can get away with 20.9 kg but if it ticks over to 21, or 22 kg, you take stuff out of the case, or you pay for the extra kilos.
It's all in the T&Cs, which if you're travelling on Ryanair, or the other budgets, especially Flybe, who are being really picky ATM, you really must read unless you want to throw money away for being tripped up. It's how they make their money when they will sell people flights for less than the cost of the applicable taxes.
If you are on a booking with other people, it's the total weight that counts, as long as no bags are above 30/32 kg for H&S reasons. So that might explain how you got away with it with Easyjet. Someone else in your party probably had a much lighter bag.
If you were allowed 2 bags into the cabin, you must have booked priority boarding, so didn't you see all the stuff about what bags you could take.
Did you pay to check in at the airport or not? The check in machines are reasonably new, at some airports they've been around for a while, but are fairly new at other airports. I agree that they are a pain, we usually have to go to the proper counters due to massive soft dive bags anyway.