Looking at the source code of the page, it seems that there is an error with the forum software Joe, tags aren't being closed in the correct order. Here's the (slightly simplified) source code on the page:
[i]true</i> equals 1 and <i>false</i> equals 0
The forum software just finds an opening i and a closing i, and anything inbetween is made italic, even if tags are not closed in the correct order.
The reason that the software does not do as you expected with the b tag is because you opened an i first, not a b. Look at this:
[b]true[/b] equals 1 and [b]false[/b] equals 0
Basically, a more sophisticated regular expression is needed to match the opening and closing BB tags correctly, however problems could be avoided if you just make sure you close all of the tags.