My understanding is that it's somewhat arbitrary. In the early 1900s, someone (or someones) decided to standardize the naming of film, and started naming the types of film at 100 (or, perhaps, 101). Since all of the designations started with a "1", the name "135" was born.

Now, that doesn't explain names like "220", which one could readily assume is just "120 is one roll; 220 is two" - or 645, which is 6 x 4.5. Aah, standards; good thing we have a million of 'em.