Uh...max Dex caps how much Dex you can apply to your AC.
If you have a +10 Dex bonus, but are wearing mithral full plate (max Dex of 3), you only get +3 AC from Dex. If you are also carrying a steel tower shield (max Dex of 2) you only get +2 AC from Dex.
If you are wearing regular full plate (MDB 1) and carrying a tower shield (MDB 2), you get only +1 AC from Dex.