When your drivers take part in races where more than one driver is allowed, the total experience allocated between the participating drivers is greater than that given to your B-Spec level. Also, the xp is not divided equally between the drivers, but allocated related possible to completed laps by each driver. Therefore if you've been using one (or at least less) driver for multi-driver races, or you normally have one driver who completes a substantial part of the race, it would be likely that you experience this scenario.
I believe this may've been in the game since the start from my observations.