Because IMO when you consider all the known facts, such as DP making no attempt to hide the fact that he called on Kate for example, then not all possibilities are equally as plausible or likely.
The fact that DP didn't attempt to hide his visit is only relevant if there's reason to believe he might have wanted to. Gerry gave the information to the PJ on 10th May and DP wasn't interviewed again until his rog interview. He certainly contradicted the evidence of his womenfolk.
1485 "Okay. So then you went back and then you played, you played a game for about an hour''
Reply "Yeah.'
1485 "You think from seven, from about seven till about eight''
Reply "Yeah, I mean it must, I mean it was before, I mean we got back before eight o' clock err you know so perhaps just before seven, err sorry, err yeah just before seven so it was just before eight o' clock. As I say when I got back I think I was cutting it fine and Fiona was still keen to go for a run and we started to get everyone ready for bed.'
http://www.mccannpjfiles.co.uk/PJ/DAVID-PAYNE-ROGATORY.htm