The PCR test is unreliable. It's just a question of how unreliable.
No test is 100%. One of the main issues with PCR is false negatives, so it's likely to under-record than over. Perhaps by 3-5%.

Imagine the Met Office with it's 200 weather Station sites decided to chase the rainfall.
This is a completely false analogy.
Rainfall is not an infectious disease. The point of monitoring rainfall is to observe an overall pattern. The point of PCR tests (in Pillar 1) is to diagnose a disease. Pillars 2-4 are to determine the overall patterning, but have not been given sufficient resources in order to function at an acceptable level.
People can't even get Pillar 1 tests at the moment.