Well, I just read this on the Glasgow site (only place in UK to do the conclusive test)
For this reason, when cats are tested for the first time, it is recommended that they be tested twice, 12 weeks apart. A very small percentage of cats which are FeLV positive are in the process of developing immunity after which they will become negative; this is another reason for testing healthy cats twice. Cats which test positive twice at a 12 weeks interval will be permanently infected. In nature, many cats which are exposed to FeLV recover from the infection. Recovered cats have no FeLV p27 in their blood but may have antibodies to the virus. There is a test for these (virus neutralising) antibodies and it is important not to confuse this test with either the p27 or the virus isolation test.
And yes, your post makes a lot of sense, and it is quite worrying when I know a lot of rescues routinely test but I think they might only test once, I am doing a bit of a survey on that at the moment.