This answer to an earlier question regarding the electrical conductivity of sulfuric acid provides a graph showing the conductivity of sulfuric acid/water mixtures ranging from 0% to 100% sulfuric acid:
(Image by Horace E. Darling in "Conductivity of sulfuric acid solutions" [Journal of Chemical & Engineering Data 9.3 (1964): 421-426.], via M. Farooq here at ChemSE.)
As can be seen, the conductivity of the solution rises smoothly from 0% to a peak at approximately 30% sulfuric acid, and declines thereafter. However, at approximately 85% sulfuric acid, conductivity reaches a local minimum, after which it actually rises slightly with increasing sulfuric-acid concentration until reaching a local maximum at approximately 92% sulfuric acid, before again dropping off, more steeply, as the concentration of sulfuric acid in the solution continues to increase to 100%.
Why does the trend of decreasing conductivity with increasing sulfuric-acid concentration temporarily reverse in the ~85%-~92% range?