Hi David,
To the best of my knowledge, "MIT License" matches "MIT:Modern Style with sublicense". Someone else on the list should probably confirm though.
The reason for the several MIT mappings is that often, projects claim to use the MIT License but then add extra language that is not the standard license text. So, there are various flavors of the MIT License. The unusual flavors are often specific to a smaller project or group of projects. Perhaps it would be better to distinguish the Allowed Licenses list to better communicate which SPDX tag should be used as the "default" MIT license. I also thought the docs were unclear for someone who isn't immersed in licensing.