Having read the light rules, each weapon type appears to have a ammo multiplier to simulate different weapons characteristics. As the base weapon size is fixed and only depends on the weapon type, the ammo of a base size weapon depends entirely on the weapon type. The figures shown in the ammo table are arbitrary and the difference between rows is meaningless. The weapon ammo multiplier for each weapon type could be replaced by the base ammo count and the ammo table replaced by a single row table of multipliers.

Think I'll stop there.