Looking over the BB1500 and BBG certs, I can see where the confusion may have come in.
It looks like they stopped the clock on the BBG in San Bernadino and stopped the clock on the other two rides in Tijuana.
The reason for the difference in mileage is different start and end points for the rides (the BBG was Calgary to San Bernadino, the BB1500 was Calgary/Tijuana, and the B2B was Coutts/Tijuana). I gues they decided there was enough difference to not consider the BB1500 and BBG to be "nested". So it look like you didn't receive both certs for the same ride, at least in the opinion of the review team.
Ira