It’s a fun question of probabilities. You get a non entirely random of 6 characters, for each you could have any of 26 letters or 8 or 9 numbers (I know for sure 0 is not used in Amadeus pnrs to avoid confusion with 0. I believe 1 is similarly omitted but can’t remember for sure).

so if for each character, you get about 3 in 4 chances of getting a letter, the chances of getting a pnr only made of numbers is very low, but the chances of getting a pnr only made of letters is not that negligible. Played the same game as cws and found that 5 of my 16 forthcoming ba pnrs are letters only.