Statistics can never simply be read from a police database or similar. It takes serious work by good researchers to compile the data and interpret it. This is also why you can't compare many crime statistics between countries, rape being a fairly typical one.
The simple truth is that up until now no one actually cared and that is why it went unreported. Heck, the new years eve sexual offences will probably disappear among all the other sexual offences that go on every day that no one apparently cares about.
Festival groping is hardly a new phenomenon, maybe the new years eve thing is new but was it a temporary problem or is it a long term issue? No one actually knows yet.
