attribute modifier vs/or xd6 rolls

[sekalo]

I just found The Fantasy Trip and noticed that some of the older documentation and rules have two forms of dice roll modifications.

option (A)
-# DX/IQ or ST


option (B)
xd6 (example 4/IQ)

What was the purpose to combine these mechanics? I have looked at the probability of the 3d6 with modifiers vs 4d6. It appears that 4d6 is +/- probability of having a -2 Attribute modifier.

In addition, I noticed that most of the skills modifications use Option B. Where the combat rolls use Option A. Only thing I can think of is that Option
B changes the bell curve.

I play a very rules light 1d6 home brew system, but wanted to add some 'lite' tactical combat. I am unsure to use GURPS and strip out what I don't want. Or use The Fantasy Trip - most Melee and Advance Melee rules with some Wizard.

Thank you