Because those damn Intel processors are taking up too much room.
In all seriousness though I would assume the difference has to do with the MBP using a higher voltage than the iPad. So it could have a higher Wh capacity (better indication of total energy available) yet lower mAh capacity.
Edit: Yup! The battery in the 2016-2017 15” MBP is 76 Wh, but operates at 11.4 V which creates a 6670 mAh capacity. The 12.9” iPad Pro’s battery is only 34.8 Wh, but since it operates at a much lower voltage of 3.77 V the mAh capacity is a higher 9237 mAh. Again, Wh is a better indication of power capacity.