I would suggest a combination of the scatter in the testing results for any given fluid, and the effect of combining multiple fluids into the generic tables.
Looking at the TCCA generic table (Table 20 in this year's HOT), abnd looking at the upper limit times, there are 13 cells where there are 100/0 and 75/25 data. Of those 13, 5 have the same upper limit, 5 have 75/25 higher and 3 have 100/0 higher. Of the 8 cases which are not the same, in two cases 75/25 is higher by 20 minutes, in 3 it's higher by 5 minutes, and the three where 100/0 is higher are by 10, 15 and 25 minutes. If you take the overall average of those 13 cases, the average is that 75/25 is better by 0.4 of a minute - effectively zero.