My reservations have always been in sequence, so in March of 2008 I had a D9 number, then it went to low E's after that, in 2009 a few E8 somethings, etc and one made last week was an F0 number, so I'm assuming they just burned through the E's and had to go to an F number.
I did look back, I made a reservation last year and this year a day apart, and in that year there is a 3.9 billion (with a B) difference in the numbers, which seems to hold year to year looking back a few years.