Yes I'm aware, but OCAN is OCAN, pay and leave is done the same way throughout the branches last time I checked...almost like there are pay grades or something. Just saying to check it dude, cool your jets.
Navy officer candidates at OCS are active duty, normally with the pay grade of E-5. NUPOC candidates have the pay-grade of E-6 or E-7 (the latter if they referred another accepted applicant prior to OCS). As an active duty servicemember, you accrue leave at 2.5 days/mo.
The language in the milpersman allows candidates to go 'in the hole' en route to their first duty station. When would something like this apply? Say you are a SWO and you graduated on March 25 with orders to your ship in San Diego NLT April. You formally get ~9 days of travel, after that you begin automatically using leave if you don't check-in to your new command. You coordinate with the ship's XO and he can support you checking in on April 29, which gives you time to find an apartment/house, move your HHG, and maybe spend a little bit of time with family before the ship owns your life. Since you only accrued 7.5 days of leave, the milpersman allows you to use negative leave until your check-in date.
Note though that it says 'may' and not 'shall.' The XO could also just tell you show up on April 10, ship is going on deployment and he expects you on it.