A bit of a confusing explanation. However, this system was in place in many European countries prior to JAR. From what I remember, the FI needed 30 hours in each type before being able to do any instruction.
Thus before teaching (or checking anyone out) in a C172, a instructor had to gain 30 hours in a C172.
15 hours seems to be a bit of a compromise but it is far better than the current system where instructors can hop into and aircraft with just a checkout but no real experience and start teaching someone how to fly it. Blind leanding the blinder!
Regards,
DFC