Importances are inherently normalized to sum to 100%.
The importance of attributes with more levels is NOT automatically higher. I've done any number of projects even in the past month where attributes with fewer levels had more importance than attributes with more.
To see this yourself, take a study with 7 levels of price for flights of $125, $135, $150, $160, $175, $190 and $200 and only two levels of a safety attribute (airline never crashes, airline crashes on 1 flight in 10). Nothing in the math will automatically make price be more important than safety.