The PBA ball speed you mentioned is in the head portion of the lanes, where the ball will have the speed you impart to it off of your hand. I'm guessing that's where the 17mph for the other bowler came from off the hand. Kegel can measure your ball speed at various points down the lane, but you typically would hear your off the hand speed because it's the truest indicator of how fast you're throwing it.
A ball can lose anywhere from 1-4mph going down lane because all of the friction encountered, since the more the ball hooks, the more it will slow down. I've noticed that on drier lanes my ball speed might be 15=16mph on qubica, but on something like shark, it'd jump to around 17.5. This is not only due to the friction, but because if you're creating a wide hook, that means the ball is actually traveling more than 60 feet and will take a longer time to complete its journey than a ball just traveling the 60 feet, even if they are going the same speed. Also, with qubica systems, if they ball is coming in at a sharp angle, it will appear to be in the path of the reader longer because it's moving sideways instead of just passing by, which will make the reader think the ball is moving slower than it really is. I don't use a spare ball i just flatten out my wrist and throw straight for spares. I've found my spare shot speed is around 19mph, but my strike shot speed, using the same armswing, is around 16-17mph.
You'd have to have your off the hand speed measured, but you can find out your average-down-the-lane ball speed by using a stopwatch to time from when the ball crosses the foul line, to when it hits the pins. It should be somewhere from 2-3 seconds. Divide 40.91 or 40.92 (can't remember, doesn't matter) by that stopwatch number, and that is your average ball speed down the lane. Perhaps the bowler you mentioned is still throwing 17mph off the hand, but if he's hooking a lot or the lanes are dry, the ball could very well slow down 3-4mph. There's a lot of variables to take into consideration, although I'd guess his off the hand speed is lower as well if he's in a slump.
Sorry for the novel, it's just a topic I've always found interesting. Especially because many people will tell you their ball speed, but it means nothing unless you know where it was measured.
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- Andy
