On the lowest level you have the radio spectrum. Channel 1, 6 and 11 does not interfere with each others frequencies at all. Channel 3 interferes with both channel 1 and 6 (with the wider 802.11g channels)

Actually, they are considered acceptable spacing because the 20mhz channels don't overlap, each channel is spaced 5mhz apart, but even when each of the three 1, 6 and 11 20Mhz channels sit side by side, there is still plenty of harmonic interference, it's just considered "acceptable".