Regardless, potential or no, unless there's some midichlorian-esque factor that can be quantified to predict the ability of Matoran to become Toa, there has to be a prior-established means of determining the statistics that they're referring to--something that even Brotherhood scientists were able to determine. An answer (whether it's desirable or not) is that at one point, there were more Toa of Light, and the Brotherhood was able to gather relativistic data measuring their potential and tendency to transform. In other words, something has to have been present for them before to understand these percentages that you refer to.
Even if it's just a matter of there being fewer of them, the question still presents itself of where the Brotherhood is getting this information.
It's a bit of a throwaway line, but it's still key to how the Brotherhood presents its activities in Karda Nui.