Hi Joanna
The US Portsmouth rating is based on results of previous races. In Europe we have a choice but the two main Catamaran rating systems are calculated not on results but boat measurements. So because the Viper is 20+kg (44+lbs) heavier its rating isn't the same as the lighter F16 104-107kg (229-235lbs) min weight.
In reality its not that big a deal but sometimes this issue can bring out the worst in people.