Answer
Arithmetic/Geometric lets you decide the Grid Spacing.
Do you want to always have the same price difference between your orders, or do you want to have the same Percentage between every order?
Examples:
Arithmetic (=Old/Normal): 100-130-160-190-….
Space between orders = 30
Geometric: 100-110-121-133,1-146,41…
Space between orders = 10%