Why? Simply because the Deliverate Stranger was made in 86, and while it has all the investigation, police insight, background and stuf, it shows the MEDIA version of Bundy: Prince Charming with an evil streak.
It was until 89 that Bundy confessed to his crimes and also to his method of getting girls into his car: beat them in the back of the head with an iron.
So much for prince Charming...
Also, Ted was much more normal BEFORE he started killing, that's when he got the prissy girlfriend, got into politics, and all that. This film is right when he starts killing, and at that point he was already showing weirdness all over. Just read The Stranger Beside Me. And even Ann Rule was astonished when Ted confessed to necrophilia, and keeping heads in his apartment, and so on.
The film shows precisely that, all that was finally discovered about him just before he was put to death, and contrasts a lot with his previous fame given by the media.
Like the lake, he was thought to be unbelievably charming to have convinced TWO girls the same day to go with him. It was later revealed he approached DOZENS of girls with the same line, and most turned him down because he had a crazed gaze in his eyes.
And why the upbeat music and casualness to the killings? The film is made from HIS point of view. No background, no police investigation, just him. The Deliverate Stranger already showed that, why repeat? For him, it WAS upbeat and funny and casual, that's why it's shown that way. And why his execution is so somber and horrible, it was for him.
Practically EVERYTHING is accurate, minus the victim's names (obvious reasons), the way he's recaptured (to accentuate how powerless he is with LIVE women), and name of his girlfriend. That's it, everything else is Accurate (well, he was missing a baseball cap he wore when exiting the Chi Omega house). Or like Ted's shoplifting antics, he DID that all the time indeed.
And the director is the one from Freeway, so don't be surprised it's loaded with sick humor. Takes one to know one...