The GI-S and GI-D have the same durability. The GI2-S and the GI3-S have the same durability as well... 2 bubbles each.
So, why do the GI2-D and GI3-D only have 1.5 bubbles of durability?
If you're going to make all 'Distance' balls the same, with less durability than the "Spin" balls, then you're going to have to knock half a bubble off of the GI-D.
If you're going to make all the GI balls the same, then you're going to have to add half a bubble to the GI2-D and GI3-D balls.
To have a mix, within each group, is just silly.
Consistency!