I'm in an Avianca Airbus A320 and the seats are numbered ABC, DEK. Any particular reason why not "DEF"?
I have never before noticed this, but a quick check over several airlines at Seatguru confirmed that other airlines do that as well.
The logic behind this is that the letters A and K will always be window seats. The letter K is chosen, because it's the highest that you can go in a normal airplane with 10 seats across (An A380 for instance). I is omitted because of it's resemblance to 1.
Note that Avianca does this even for the smaller airplanes, where you'll have ACDK. B and E are omitted so that C and D are always aisle seats. This is also maintained on the bigger A330, where numbering is AC - DEFG - JK.
Just to illustrate, here's are the seat configuration for all their planes:
So A and K are window seats; C, G seats with aisle on the right; D, J seats with aisle on the left; B,E,F middle seats.
It's a little illogical on the other side though, I don't understand why they use J instead of H, would make more sense to put a H, then if they happen to buy bigger air planes, they could just fit in the B and the H.
There's a little bit more information on this on Wikipedia.