For drag race, there is a 20% chance that one car will breakdown at the starting line (turns grey)
For the remaining cars, there is an equal chance. Since both these events are mutually exclusive, the odds turn out to still be 12.5%
Funny thing about stats is that the probability assumes an infinite number of events. With a smaller event set, it is entirely possible for your "sample" odds to appear different.
For example, take a dice. Probability states each side has a 1 in 6 chance of coming. So if I pick a number, and throw the dice 12 times, it should appear exactly twice.
If you try that out, you get a very different result set.
I actually setup a spreadsheet to do exactly that. Generate a random number between 1 and 6 twelve times. This was the result.
Now I increase the sample size to 120 tries. This was the result.
With 12000 tries, this is the result.
As you can see, with small sample sizes, the probability 'appears' to look skewed....this is what is happening with your tests.