One of the things I noticed when I was going through the photos from the event last Sunday was that for about half the time, I had panned with the horse brilliantly. The other half (primarily when I got tired at the intro/pre-intro, but sometimes when I focussed on the jump - more on that later) were like many of my other photos - the background was stiller than the jumping horse. It shouldn't have made a difference, but it did.
The photo of the grey is actually at a slower shutter speed than the bay (1/320ths of a second compared to 1/400ths of a second), although looking at the sharpness of the horses I would have ranked it the other way around!
While at higher shutter speeds such as my normal 1/800ths or 1/1200ths I doubt this would matter, it's certainly something I'm keeping in mind for the future (as common sense as it sounds!)
The reason why many were panned was because I was prefocussing. Manually changing my AF point to the centre point (my camera's centre point is more sensitive than the other points), I focussed when the horses were about three to four strides out wherever possible and followed them over the jump. One thing I'm prone to is to take the photo and jerk the camera up, running for the next jump, so many of my photos have the bottom missing. To try to stop myself from doing this, I made myself continue panning to about two strides away from the jump, which is how I ended up getting the better photos. In contrast, previously I relied on the AF-servo mode with all 9AF points or (even worse) prefocussed on the jump - meaning that the jump was perfectly in focus, while the horse was slightly out for most of the time).