Are you flat at that time? If not, who cares. If you are only putting in as much throttle as the tires can handle, it doesn't matter that you're not making full power. You watch the tach on a lot of high powered bikes, they are way down in the RPM range, but since they'd be spinning the rear wheel w/ more throttle, it is faster to stay in the higher gear as you have fewer changes down the straight.
I don't discount that it is odd. However I would want to see on the straight how long it takes to equalize when you go from partial throttle to flat, squeezing it down, not making the change quickly. When you do an upshift, you go from flat, lift, flat very quickly. The motor has been off the throttle for a bit at the turn and is probably at neutral throttle, or low amounts of accel prior to you going flat. This difference may be why you don't see the problem on upshifts.
