In Anima, fumble can only occurs on a natural 01-03 (01-05 with disadvantage).
The rules about fumbles are quite easy (as described in Core Exxet - p.007-008):
When the player rolls a natural 01-03 the action is automatically failure. You then have to proceed to the level of failure by rolling a non-opened D100.
For an opposition roll (offense action vs defense action, search vs hide, etc) you substract the level of failure to your skill. In the case of initiative, you substract a specific value (Core Exxet - p096-097).
The +15 / 0 / -15 in the first book where added to mitigate the level of failure. (so added to the level of failure roll). For example:
- Bob has a Bastard Sword skill of 100
- Bob roll a 01 on his attack. This is a fumble with a +15 modifier.
- Bob roll a 90 level of failure.
- With 1st ed rules, his attack roll would be: 100 - (90 + 15) = 100 - 105 = -5
So Bob final attack roll is -5.
Now your case:
- Alice has a Bastard Sword skill of 100
- Alice roll a 03 on his attack. This is a fumble with a -15 modifier.
- Alice roll a 90 level of failure.
- With 1st ed rules, his attack roll would be: 100 - (90 - 15) = 100 - 75 = 25
So Alice final attack roll is 25.
New case (totally corresponding to question case): ww
- Alice has a Bastard Sword skill of 100
- Alice rolls a 03 on his attack. This is a fiumble with a -15 modifier
- Alice rolls a 5 for level of failure.
- With 1st ed rules, his attack roll would be: 100 - (5 - 15) = 100 - (-10) = 100 + 10 = 110