As much as in theory your last paragraph is how it should work we all know that the majority of people vote based on which party they represent. if they didn't far more Independent candidates would win.
There are safe seats like where I live where it doesn't matter who the party puts forward as a candidate, they will always win purely because they are a Tory. In this case, after the last MP retired, the Tories shipped some young Venture capitalist from down south which nobody had ever heard of and he still won (despite being the son of an immigrant and people round here hate immigrants"