That's because the 220V AC is the integral (as in that math you learn in high-school) part of it. That 220V AC outlet actually has 630 V peak-to-peak voltage difference. That's why AC voltage is riskier than its DC counterpart, but that's because the numbers for AC are somehow misleading. Anyone who ever build an AC-to-DC converter knows that after you run your AC source through a rectifier bridge your get 315V at its exit. Then you use capacitors and maybe induction coils to "smooth" it, and that smoothing is what gets you your 220V DC source now.