Battery pack capacity confusion. Capacity rated at 3.7V but outputs at 5V?

I just came by an interesting question about portable battery packs.

This one product is advertising at 10400mAh at 3.7V with 38.5Wh capacity.

However the product outputs at 5V.

Therefore with the 38.5Wh battery at 5V output, wouldn't the overall capacity be 7700mAh?

Why quote the capacity at 3.7V when you output at 5V?

Enlighten me please.
Asked Apr 23, 2014

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