Understanding Rule 17 and 18

Okay. I've been reading these rules, and I'm not entirely sure that I understand them. I'll run through a hypothetical situation, and those that do understand can say what I've read wrong.

Any player can make a budget proposal, which looks like: Defense 30%, Public Works 30%, Reconstruction/Welfare 20%, Industry/Business 20%. Once a week, the Speaker picks two, and we vote on them. If the above was passed, and my preferences were 50/20/20/10, then I would lose 20 Credits in tax, and gain 3*3+2*3+2*3+1*3=8*3=24 Credits in Returns.

If that's so, then what's to stop people changing their preferences to fit the budget suggestion that looks like winning? And is is really possible to lose credits through this?