Well I don't understand using one 1.5 rocker? They don't cost that much to buy a new one?
It's not unheard of to use 1.5 on intake and 1.6 on exhaust to compensate for head flow, or even to use them in reverse for the same reason. But to use one 1.5? there is no reason. cheaping out, lazy or who knows what was going through the builders mind.