Since you have the credit AVAILABLE to you. An underwriter has to take into consideration what you can afford IF you were to take all of the credit available to you.
Just as an simplified exaggeration to make my point. Lets say I have one credit card with $250K available credit. I owe nothing on it and have no other debt. Say I make 40K annually (I don't
) and I apply for a mortgage. The underwriter says ok this dude can afford a payment of $750 a month (just guess) but if he uses all of that credit card there is no way he could afford the house and the card payment. See?