It's because of the space the numbers take up on the bezel. Also, if you count from the '0' pointer on the bezel to the first mark, you will see that's two sixtieths, thus 5 spaces between the pointer and the middle of the 5 digit - same all the way around. From a diver's perspective, you would either: set the bezel to the minute hand at the point of descent and time your bottom time to a 5 metre, 3 minute safety stop; or, set the bezel by planned dive time, eg, 35 min dive - set the bezel 35 min marker to the minute hand, then "dive the plan" and again a 3 min safety stop at 5 metres.
Probably haven't explained that very well
