Your Montana Form 2 begins with you including income and adjustment items from your federal tax return. Then, state-specific additions (and subtractions) may be required, in which case it's possible that your Montana taxable income reflects what you're seeing.
See Montana Additions Schedule in the Montana Form 2 Individual Income Tax Instructions to see what additions might be affecting your return.
If this doesn't address your issue, feel free to post back with more information.