Hah--I'm not certain about the kind part, but I do love a mystery!
The test5 file is using the Quark Generic gray.
The test6 file is likely using the stock PDF/X-1a profile and the output condition uses the US Web Coated (SWOP) v2 profile. Which is probably why the 100% K held true.
The oldest version of Q I have installed is version 126.96.36.199. Using either PDF/X-1a or my grayscale profile as shown above, all I can get is the 100% K. As well, if all I do is choose to export and open the Options from the export dialog and choose the Gray 100%, the 100% K holds.
However, if I choose the plain Grayscale, I'll get the 90% black. The reason, I believe, is that it it accounting for 10% dot gain. Might be semi-useful for newsprint, but not for typical book production.
Because I don't have an earlier version than the 9 series, I cannot check to see if the output styles are the same, but if I recall, they've not changed in several versions.
The short of it is this. Only use the Grayscale 100% output style to make your PDF output profile, or make sure to choose it in the PDF Export Options.
I prefer to make specific PDF output profiles. If you also have one to use on the books, make sure to edit them to choose the Grayscale 100% output. Save them, choose them from the Export PDF window.
Best regards, Mike