At character creation you get three options.
1. One Expertise die of d8 in one field.
2. One Expertise die of d6 in two fields.
3. One Expertise die of d4 in three fields.
Option 1 is for an academic type background. A character who is really committed to an area of study. Someone focused on being the best blacksmith or the most knowledgeable herbalist or a foremost expert on gnome yodeling. Whatever the area, there's a singular focus and therefore a better chance of pulling up a useful tidbit.
Option 2 is for the somewhat educated or experienced. Option 3 is more of the jack of all trades or someone with varied interests.
Why change it you ask?
Because a set in stone +10 makes everyone an automatic expert. A Nobel laureate in two fields.
Also, there's already the handy precedent of the Expertise die. Use that sucka!