Baggy T the rating system is silly but it is based on the ratio of black squares to white squares. If there are not many black squares it will be 5 star rating. If there are lots of black squares it will be 1 star. Stupid system as it doesn't actually correlate to difficulty.
Robaharrison - Thanks for that comment. That a pretty unreliable system. Difficulty is more based on the number of filled-in squares to non-filled squares in each line... but I suppose that's harder to program!