Maybe they had your parents mailing address as your "garaging address". Mailing address could be different then then the other. The garaging address is where your car is physically stored when you are not driving it. Insurance "zones" or in nationwide's lingo is county and territory. The zip code is part of it but not all. Basically you have the zip code, then a county code, then a territory code. It is possible the same zip has a different territory code. This is how it considers risk while the car is sitting by itself.
If your mailing address was wrong it could of also been the garaging address as well. Then they changed your "address (mailing and garaging)" which could of put you into a higher risk area.
Also there could of been a rate change after your policy renewed "mid term". Rate changes just happen to keep Nationwide from losing money. Sometimes they reduce premiums by a percent but most of the time they raise it. All insurance companies work this way. Between these 2 that could of raised your premium.
Also you noted Dec 07 your tags were expired? Did you get pulled over and get a ticket for having expired plates? If so this is considered very bad and will automatically put you into a worse company. This almost always causes a premium increase. With nationwide it takes 5 years for this to fall off your record. And to answer your question previous I have seen MVR (your claims history report) not display a violation until a year or so later. It is rare but it can be an issue between the police and the DMV.
Shit happens dude.