Retroactivity makes the accrual mathematically awkward: until the threshold is crossed, the correct accrual depends on the probability of crossing it. Most businesses accrue at the currently-achieved tier and take a catch-up adjustment on crossing, which is defensible but makes the liability lumpy.
It also makes purchasing behavior discontinuous near the threshold — which is precisely the behavior the supplier intended to create, and precisely the behavior you can only exploit if you know where you stand before the period closes.