I understand it, you have to put the things down in the right order to get everything to work together. 20000 is the best possible score.
Some things can affect others, and every time you put something down everything else already down has a chance to level up.
An example of the affect is if you put the spinning thing down first, then later put down the pipe, when the pipe levels up and tries to make clouds they get blown away by the spinning thing.