I was as flummoxed by this as you were until I went to the search page for the Probate Indexes, scrolled down, and saw this:
Our collection covers 80 years from 1861 to 1941. We currently do not have the books for the years 1858-1860 and there are some gaps for the years 1863, 1868, 1873, 1876, 1877, 1883, 1888, 1899-1903 and 1910-1911. However, we hope to add records for these years as soon as possible.
The years 1899-1902 are actually completely missing in the "browse this collection" drop down menu, top-right of the search page.