I won't bore you with the "Whys and What For" but it is related to the direction of the light source on the Green channel of the normal map when it was created and this can differ depending on which program was used.
It's quite common for a Normal map to come out "inverted"
The good news is, it's also quite easy to fix in any 2D program.
Just follow the steps below:
|CLICK TO VIEW. - STEP 1|