Eina!. Gunns I owe you one!!
The Comair aircraft do of course carry heavier loads and no, it has nothing to do with the weight of the pilots either!
At higher airfields elevations like JHB and WHK with long runways you are given an option in the T/O performance data to use a set of parameters which is called the Improved Climb Performance data. This in short allows you to lift more weight by increasing your V1, V2 and Vrotate speeds and still make your climb gradients as required by law. This obviously increases your T/O roll distance - It is the penalty you have to pay. This is obviously only nessasary if your weight is above a spesific limit because you do put more "pressure" on tyres and should a T/O rejection occur at V1, it is guaranteed (spelling) to
.
To answer your question - The Comair aircraft probably had a heavier load and was using Improved Climb data whilst the other aircraft was using normal performance data.
Maybe the Boffins would like to expand on this subject.