The advertized IBO speeds are for a bare bowstring and the birth certificate states the speed of the bow before it went into the box before shipping. All BowTech bows that I've tested were very close to, or exactly what was on the birth cretificate. Chronographs can vary so the speed you see may be due to chronograph differences, lighting or where the arrow passed through the screen of the chrono. I've seen quite a difference on my chrono if I shoot accross the screens too high, usually about 3 to 4 inches above the top of the chrono will produce consistent results from week to week to months between testing the same bow. The chrono needs to have the proper amount and quanity of light to read the shadow of the point of the arrow as it passes above the screens. I've had my chrono for about 12 years and it's been as dependable as ANY of the best of my hunting and shooting gear.