The appropriate measure of capacity would be watt-hours, not milliamp hours.

IIRC the MacBook Pro has a 3 cell battery, meaning that you'll get around ~76Wh from the battery (11.2V * 6800mAh).

It's not safe to assume that all laptops will have a 3 cell battery. They might have chosen to install a 2 cell battery, in which case the capacity would only be ~50Wh (7.4V * 6800mAh). That's 35% less capacity!

True. But is anyone shipping laptop with a Voltage that low? I see them up around 10.8 - 11.1.

I don't know about laptops, but I would expect a lot of these "2 in 1" devices to ship with a 1 or 2 cell battery.

Tablets frequently have lower voltage batteries. The iPad 4 for example has 3 cells but they're all in parallel, so the voltage is only 4.2V.

For laptops the sweet spot seems to be 3 cells, and I'm not entirely sure why, apart from higher voltages requiring smaller traces/wires. Maybe due to the LCD backlight? The LCD from the iPad 4 actually requires ~12V for the backlight, so there's a buck boost converter somewhere on the iPad logic board for this.

Ah shit I was thinking of WHr which is different.