My guess would be that sometimes instructions are written for multiple model numbers, and instead of having separate instruction pamphlets for each model of timer the manufacturer just uses the one multipurpose instruction pamphlet. I would be willing to bet that the 15 amp model doesn't have a neutral wire for whatever reason and the model that the OP has is probably the 20 amp model.What the instructions say about the neutral has me scratching my head, don't know why you wouldn't hook it up to a 15A circuit as well.
As darren stated your connections would be:
black - (2)blacks
blue - black
white - whites
green - cap with wirenut