Lucky Driving

"Nine is considered a good number in Chinese culture because it sounds the same as the word "long-lasting". Hence many people want their car registration numbers to sum up to 9 on adding the digits recursively. A car registration number can consist of 1,2,3 or 4 digits. Examples of some good car numbers are 63, 018, 9099, etc. whereas 12,129 are not good. Why is 9099 good? 9+0+9+9 = 27