Comma Splice: "Tom opened his freezer, then he noticed that two cans of beer had exploded." This is a comma splice. It has 2 independent clauses.

" I cannot afford a house in London, so I am moving to Sunderland." and "The computer in my room is old, but the carpet is ancient." But why sentences like this are correct? Don't both these sentences also have 2 independent clauses? Because in the first sentence I is the subject and moving is the verb. In the second sentence, the carpet is the subject and ancient is the verb. Help please.