If the tests aren't capturing actual cases, what the hell are they capturing??
They are capturing a percentage of active cases.
According to both Zoe app and ONS, we are now running at around new 3,600 cases per day. If the testing only picks up 1,300 of those, we are still missing a meaningful proportion of infections.
Ideally you would like the two numbers to converge but it will not be possible due to fully asymptomatic cases.