I would suspect that the lower incline was driven down through the floor of a level.
Might need Graham's input on this one but my suspicion would be that this was done to gain another access to bring slate upwards as work progressed not new territory under Cwmorthin.
If my memory serveds me correctly, there are another couple of inclines in the combined workings where the lower part of the incline is of a much smaller bore - if my memory serves me correctly, the lower section of the OV incline and the (abortive) continuation (a chamber over) of the BV incline, although there may be at least one other.
It may be that this design reflects later working practices, ie that inclines were used solely for transport whilst the movement of people was a chamber or two over; whereas earlier the inclines were built of a larger cross-section as they were also intended to be used for foot traffic.
That could be down to increased reliability of inclines (ie the space to re-rail was no longer such a necessity), elves and safety (prevent people being crushed on inclines) or merely that whoever was in charge had different ideas - which I'd probably favour.
One further consideration might be that there would have been no need to maintain an access for people as this would have been provided much closer to the Oakeley pit, but that doesn't negate the requirement to move between floors at that particular end of the quarry - and given that management was concerned, I reckon they'd prefer not be be squashed!
Thinking on, there are various stairways in parallel to the narrow-bore incline in that area of Oakeley, so there may be some credence in the idea that someone had decided that people and inclines didn't mix - and in that case there was no need to have full-size chambers to run the inclines in.
Conversely, the rock around the incline was probably perfectly useful had it been quarried, so it's slightly odd why it wasn't...
At this juncture, I fear I'm starting to go round in circles?!
Hello again darkness, my old friend...