Nothing undesirable happens immediately as a result - except that maybe your manager is a little bummed out. But maybe the next time one of these projects comes

Nothing undesirable happens immediately as a result - except that maybe your manager is a little bummed out. But maybe the next time one of these projects comes up, they’ll ask Bob first. Maybe that speculative project turns into a full fledged project and you missed your chance to work on it. And when the over-committer Bob ducks out of the project due to time constraints, the manager will come back to you, knowing you have a good track record of completing tasks.