As was stated it is a standard used to measure a bows speed. IBO speed has nothing to do with a 31 inch draw.

IBO speed is always tested as follows. It is a standard used to rate a bows speed as horsepower is used to rate an engines power for an analogy. You never deviate away from the test specs for a bow just like you don't when testing an engines power.

70 pounds
30 inches
350 grains.

Except for childrens and ladies bows that do not draw 30 inches. They are then tested at 5 grains of arrow weight per pound of draw weight for their speed.

A longer draw is just on top of the IBO speed. Kinda like your car was rated at 200 HP then you add a new mainfold and now it has 210HP.

I hope that was an OK analogy. Probably not though.