Your probably right about the doak odds. How those things usually work is they take a seed or two for a pseudo-random number generator. In this case I believe they use the frame you leave the vent as the seed. Then it makes an integer, usually very large. Then they do the mod function on the integer which divides it by some number and gives you the remainder. So one might think they would do mod 6 and get an integer between 0 and 5 for 6 different equally likely events. However, I found that many programmers like to make the events have different probabilities. So they would do something like mod 20 and get 20 different results then assign 5 of them to some locations, 3 to another locations and maybe 2 and 1 some too. So you end up with odds like 5/20, 5/20, 3/20, 3/20, 3/20, 1/20 for the six different locations.
If you can hack the rom it isn't too terribly hard to find the code which tells you the odds.
