It's intentional, though I'm not positive of why.
My guess is to make it better then using stright refinement.
Direct refinement bonuses are +2 per point of refresh, but must be supported by lower bonuses.
Focus slots are +2 per point of refresh, but are stuck in objects and can be changed in downtime, and you can concentrate them where you really want them.
+10 takes 5 refresh slot as Foci, +10 as a innate refinement takes 28 refresh slots (starting from nothing) and comes with a +9, +8 and so on assigned to other things. In theory, given 5 elements, offensive vs. deffensive and power vs. control, you can only hit +20 at most. If you can somehow (ha) get a lore of over 20, you could get a Focus that gave more then +20 too...