Convention. The Lee Enfield's 10 shot magazine is really a 5 round magazine which can have 10 crammed in but does not do it too well.
Loading in the second 5 found charger is not smooth and slows you rather than just firing five and loading five, and as I said the rounds don't actually feed so nicely from a full 10 round magazine. So he starts out with 10, fires 10, and then loads 5 at a time.
The original mad minute I believe you start with 10+1, fire 6, [5 left] insert 5, fire 6,[4 left] insert 5, fire 6,[3 left] insert 5, fire 6,[2 left] insert 5, fire 6, [1 left] insert 5, fire 6... and you have an empty rifle and have fired 36 rounds.
In combat you'd start out with 10, and fire away, any chance you got you'd drop another charger of 5 in. The idea of the larger magazine seems to have been to ensure that there (usually) always will be space to take a whole charger.
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