I think test and trace only ask who you've met going back as far as two days before you got symptoms, because they're looking for who you could have given it to. So basically if you go out for the weekend and develop symptoms next Thursday, the system only wants to know about Tuesday onwards. They don't ask how you think you got it (which could be the pub, the gym, the cafe, or even the church 4 or 5 days before.
So even if people are doing it "by the book" and being completely honest, a lot of people genuinely won't have been anywhere or met anyone outside home, work, supermarket.
I think it's a major anomaly, because it means that they don't get much epidemiological information from the system. Nor can they alert people who are asymptomatic that "last Saturday in the pub you may have met a superspreader - get a test." Of course, delays in the system often mean it's pretty irrelevant anyway - one of my family has just been advised to self isolate for two days.