The roll to hit is a single die, so if the attacker could hit or miss on a regular (1-19) roll, the best outcome is to block the lowest number, AC+6. The extra number gives no benefit against an attacker that couldn't roll that number on a (modified) ordinary roll, and gives a 5% miss chance against an attacker that could.

If the attacker has such a high chance to hit that they can roll AC+15 on a regular die, but cannot roll AC+6, you're in trouble - they'll basically never miss.