Fixed in an internal build
There may be the occasional combination of selections that show varies when you might think otherwise. If multiple selections are made, we populate the list of materials from the assembly document (includes materials that are referenced in the Style library, if used). If the part document(s) contain additional or different materials, and one of these is in use, we show varies since the material cannot be located in the list.
We originally tried to merge materials from all selected documents for the list, but it crushed performance. If a single document is selected, the list is populated from that document.
I'll include the fix in the next update posted to the site (a day or two from now)
Thanks.