... With strobes (eg fluo lights) would you not expect a series of equally lit images if the ball is traveling at the same speed throughout? ...
The short answer is that as a result of the pulsing flourescent light, this is effectively a "quintuple exposure".
The image created by the basketball isn't made by light reflecting off the "subject" (i.e. the ball) so much as by blocking the light that reflects off the back wall, and so we're creating a silhouette. That light isn't constant, it's "pulsed" by flourescent lights, much like a strobe light. Looks like about 5 pulses during this exposure.
For the first pulse of light, a perfect silhouette of a basketball was created. The silhouette created in this first exposure is uniform and dark. If the shutter speed was faster, we'd see this perfectly uniform, dark, opaque, silhouette of a basketball instead of a smear.
For the second pulse of light, the ball had moved a little bit, so some of the area that had been blocked by the ball in the first shot is now revealed, allowing light from the back wall to pass through. This topmost sliver of the basketball "smear" will also be exposed in all the remaining pulses, so it will be exposed in 4 out of 5 of the pulses, and so will appear almost as bright as the background.
For the third pulse of light, the ball has moved further. The second sliver from the top will now be revealed for the first time, it was blocked in the first 2 pulses. This sliver will be exposed in 3 out of 5 of the pulses, so it will be a little less bright than the topmost sliver.
And so on...
The bottommost sliver of the basketball image was exposed for the first four pulses and only blocked for the last. It will be fairly bright, as bright as the topmost sliver.
The middlemost slivers will be the darkest as they were first blocked by the bottom of the ball, then the middle of the ball, then the top of the ball, and so the light from the back wall was blocked the entire time, so this area is the darkest.
-harry