You can always choose not to use full BAB or any other ability (perhaps with some exceptions), it's ridiculous to say otherwise. Are you telling me a pro basketball player can't choose to miss a dunk? Whether or not you can convince someone that you're actually trying your hardest while you do so is another question entirely. That requires a bluff check.
[edit] Auraseer - you bring up a good point... but there's a problem. It might become obvious from the way Robin shoots that he's really skilled, and just choosing to miss. Skilled bowmen can probably tell when someone else is really good... the way they breathe, how they hold the bow, how they fire an arrow, etc.
The only thing I can compare it to is the only physical thing I'm really good at - foosball. I can generally tell in about 5 seconds if someone is good, regardless of how often they score. You could probably fake being sucky, but an instinctual response could easily give you away. To properly fool someone for a significant length of time would take a bluff check.
-The Souljourner