Ok this is the number you speak of, when you hover over it, it says weapon mastery. This is your attack percent, when this number is lower it means you are attacking a target smaller then you and get a penalty. If you add an officer to the 100% it will show at 110%. Now if you start with a red percent like 35% means that is what you will attack that target at, even adding the Grandmaster will still only put you in the red at 45% attack strength. This is of course in province.
When you attack out of province of course many factors come into play on that number, do you have a church in the province you are attacking into? Are you using your pally with level five statue with the troops? Is your target smaller than you?