Or possibly that the criteria used to authorise a test in March produced a high positive rate, while the current criteria, testing everyone in a hospital, say, produces fewer positives?
We don't know, and it doesn't help to pick those speculations which suit your wishes.
I think that we can now assume that the data from every country in western Europe are accurate and the differences in the ways they are collected are understood. It is quite clear that the UK currently has a higher rate than in our usual destinations.