Note the aircraft diagram included...a B747 following the A306...2.5nm minimum wake turbulence separation.
Sorry, perhaps I'm being obtuse, but I don't see that.
The diagram shows wake turbulence separation of 4nm applied when a A306 (RECAT Group C) is following a B747 (Group B).
But when the B747 is following the A306,
no wake turbulence separation applies (RECAT rules same as ICAO), so the 2.5nm (minimum radar separation) applies.
In fact, looking at the wake element of the RECAT matrix, there are no combinations of leading/following category that result in a 2.5nm separation on wake turbulence grounds (nor are there in the ICAO matrix).