I think that it may have something to do with making it easier to reassign seats in case of equipment change. Rather than a garbled explanation call up the seat maps for BA 747 Hi & Low J on seat guru.
SeatGuru Seat Map British Airways Boeing 747-400 52-Bus. (744)
SeatGuru Seat Map British Airways Boeing 747-400 70-Bus. (744)
You will see that there are no rows 6 - 10 so that a row with single a digit is first.
In the Low J version there are no rows 21 - 27, in the High J version only row 32 is missing. This means that rows 17 - 20 are always J and 28 - 53/55 Y. There are no rows 56 - 59 but when a FA sees rows 60 upwards they know to show the passenger upstairs.
I've probably not made myself clear so have a look at the maps.
Northwest DC9s used to have ACDE&F but no B. Thus if one were substituted for a 727 / 320 you would only have to reissue boarding passes for B seats. You know that A&F seats are window seats and C&D aisle if flying on a narrow body.
I would imagine that the reason for Varig's lettering is that firstly I can be confused with 1 hence HJK. Then then know that A&K are window seats, C&H aisle and so on. DEFG will only appear on wide bodies.
In my experience the American's have a greater fear of the number 13 than the Europeans and you will not have a flight 13, 13nn, nn13 etc, no Gate 13 & so on.
I presume that you still get a boarding card sequence 13. Life could get rather confusing if it is excluded.
The fact that this is my post 113 is pure coincidence.