The JAA considered that the twin is a more complex aeroplane and together with Types, should be checked annually. In reallity the JAA do not see twins as PPL aeroplanes.
The SEP revalidation was meant to be a bi-enial flight review as per the FAA system. Certain countries wanted to revalidate by experience instead, hence the either/or system of a check, or 12 arbitary hours in the latter half of the validity period, plus a dual flight to make up for the lack of a test.