I think in some restaurants, it is the certified server's choice whether to share the tips or not with the person s/he is training. The more generous people will understand that you did all or most of the work and give you at least some of the tips, while others may not share because they figure it is a out of their way to train someone. I've only worked at one restaurant, so I don't know. The head server would keep the whole tip for herself after shadowing the trainee, but when another server had to train someone, he was considerate and let the person doing the actual waiting take the tips.
Edit: What I mean by 'out of their way' is if they don't get their own tables yet have to show up at work for only some cents per hour, it would be a bit of a sacrifice to them to spend that gas money on watching other people make money. I think that might be why some people reserve those tips for themselves. I really do not know.