Air Strike calculation - why doubled?

[Sir John Cope]

Probably a stupid question, but I have to ask:

The user manual says "the attack value of the air unit is doubled and then standard Combat Results are applied to the target unit"

Why?  Why not just double the values in the OOB?  

Or is it that this applies only in the specific condition with which the relevant paragraph begins: "If there is more than one target in the target hex"?