I always felt like 13 is the quickest to go because it compensates for 3 different size ranges. 12.5 (wide), 13, 13.5 (narrow) .
People who have a wide foot and WOULD be a 12.5 and people with a narrow foot who WOULD be a 13.5 go with a 13 since such sizes are, generally speaking, not available; therefore, they opt to go with a 13 to have their best fit and deal with a little less or a little more length and then you have the actual people who wear 13s. However, despite this I doubt that there are 3x as many 13s being produced to compensate for this occurrence. Im just guessing though. Either one of could be righ or wrong or whatever.