Perhaps a "dot dot" version e.g. 9.9.1 does not contain changes that merit its having a updated MIB?
I notice that, for all the preceding ONTAP versions listed on that page, there is just one MIB for each "dot" version e.g. 9.6 has mib_2_10, 9.7 => mib_2_11 and 9.8 => mib_2_12. The MIB versions have always incremented linearly i.e. by 1.
On the other hand, there is a difference for the newest entry. It has "... MIB for ONTAP 9.9.0" in the title rather than just "9.9" with a single "dot", as all the previous entries have. But maybe that is just misleading?