anecdotal tales of multiple -ve LFT’s followed by a +ve PCR.
My understanding is that this is, in part at least, due to the effectiveness of the vaccines.
Apparently vaccinated individuals who come in contact with Covid get an immune response very quickly, much faster than those unvaccinated, as a consequence they show/feel symptoms when levels of the virus are still relatively low and not high enough to be detected by LFT.
By the time the unvaccinated start showing an immune response/symptoms the virus has reached relatively high levels and is more likely to be picked up by LFTs.
Obviously because PCR tests are considerably more sensitive they will detect lower levels of the virus. I also suspect that the fact that PCR tests are generally taken at a later time than LFT might a difference, things can change dramatically over a few hours which is why it is recommended that LFTs are done at the last moment before coming in contact with vulnerable people.
Edit : Another reason I would have thought likely is that apparently many LFT negatives are due to badly carried out tests, the PCR tests take at least part of the process away from the untrained.