1, 23, 18, and 26 is shared with OC0A and OC1C so it can be either 8 or 16 bit, which is where the 15 or 16 PWM pins come in. There are 16 pwm pins, but there are only 15 physical pins, 26 being shred gives you either 4, 8bit or 12 16 bit pins. But I'm not sure if both types can be used at the same time.