If you leave in 6 months time on a 12 month contract you have to pay the other 6 months.
If you leave in 6 months time on a monthly contract you dont pay additional monthly fees.
The other fees you are paying have nothing to do with monthly, annually etc contracts. They are connection/modem/migration fees. Which all customers I think sign up to pay off over a 5 year deal, so although you sign for 1 month or 1 year if you leave before 5 years you have to pay some fees. This all depends upon what package you got when you started (free modem for example) etc.