Carolyn,
It could be Brother changed the bobbin case tension so 60 wt can be used and it is the older models that use 90wt. It is also possible the manual and website have not been updated.
If the 60wt works, it works!
Babylock came out with the Emore, an embroidery only machine which was the first EM to use 90wt bobbin thread. At that time only BL had 90wt, Finishing Touch bobbin thread.
Now more vendors have this weight thread. http://www.icanhelpsew.com/janome-bobbinthread.html
Embroidery only machines normally use class L bobbins.
Combos usually come with class 15 and to use the prewounds, a bobbin center pin had to be used because the prewounds were L only and shorter in heigth.
Now, some vendors have prewounds in class 15. http://www.allstitch.net/department/prewound-machine-embroidery-bobbins-10002.cfm?killnav=1
Some Emore owners have requested their service tech change the tension on the bobbin case so they could use the 60wt prewounds.
Pattiann