I have two different saved games and the requirements for the star levels are different. 

Example - Save 1 - Pass water Cove, I am on 19/22 stars and the requirement for 20th is 33,139 buzz

Save 2 - Pass water Cove (newer game run through), I am on 16/25 stars and the requirement is 63,453??

Why so different - should they not be the same?

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They're the same - up to level 5.

If you get so much buzz, or reach a high level at any stage then that's going to be the benchmark for future stars, and if you've already maxxed out as - say 12/12 stars, and keep adding buzz and/or levels, then future requirements are just going to be that much more difficult. Sometimes you'll even find that once you've reached the next curator class, just revisiting a museum that was previously maxxed out will instantly award a star, without you ever being aware of what requirements you'd already reached.

If you have (or make) a save just before a museum levels up, and you don't like the options for the next star, then you can edit the save's name from your backups and reload ... you'll get usually a different requirement for your next star.