I bought a vacation house in 2007.. the last 3 years i rented it out. i sold it in 2020. why does turbo tax show my cost basis as different than what i paid in 2007?

you took or were required to take depreciation for the periods it was rented.  if you didn't you would need to file form 3115 to catch up on the depreciation you should have taken.  some taxpayers can do this n their own others use a pro to make sure everything is correct. for example, the land portion of the home is not depreciable.  if you made improvements they need to be depreciated also.