Used 6 Div Orbs on a spear and the flat phys didnt change

Im confused, maybe I dont know how div orbs work?
The spear has 17-28 phys on it with a possible roll of (16-24)-(28-42)
After 6 div orbs it changed 1 time on that specific multiplier to 17-29 after 3 or 4 divs, then went back to 17-28 for both the 5th and 6th. Is there is % chance it doesn't change or something?
I think there is no way that its just bad luck that 6 rolls never changes a roll. Is there a bug?