In the Open Type spec one can read :
"If bit 8 is set in OS/2 table v. 4, then ‘name’ strings for family and subfamily are provided that are consistent with a weight/width/slope family model without requiring the use of ‘name’ IDs 21 or 22."
The 'name' table for the "Gabriola" doesn't have the name ID's 21 and 22. The font "Arial" also doesn't have these name ID's, but it doesn't have the bit WWS set, in the OS/2 table. What's the difference between "Gabriola" and "Arial", regarding this bit ?
As a matter of fact, I just found out that, among the ~190 fonts used by MS Word, the font "Gabriola" is the only one that has this bit (WWS) set.