In the link you provided it says there are 2 +12v rails, each says its 18A. Below those is a wattage 312 which is for both rails.
Each rail can provide 18A on its own but they both cant provide 18A at the same time 18+18 =36. This is because there is only 312 watts available to both rails to share, the math is, Watts divided by Volts = Amps. so 312 divided by 12= 26 A shared across both rails. Some PSU's can provide enough for both rails to provide the stated amount but quite a few cant like yours.