Yup you are correct, 15 seems arbitrary though i don't know why that was chosen. max CL 15 means that you cant scribe 9th level spells (which it just sets the min to whatever is hard coded but something over 17).
"Benefit You can create a scroll of any spell that you know. Scribing a scroll takes one day for each 1,000 gp in its base price. The base price of a scroll is its spell level × its caster level × 25 gp. To scribe a scroll, you must spend 1/25 of this base price in XP and use up raw materials costing one-half of this base price."
and from the wiki " You can choose the CL by way of an input window that pops; there are certain restrictions like you can't go below the spell minimum or beyond your CL."
So we might want to update those as well as the feat description: "Any arcane or divine spell of any level may be placed on a scroll." Or its a bug.
Though I also haven't tried to go over CL 15 on a scar yet either. And another side note that you can only one one scar per spell level max 6.