I took a prospecting mission, left station, targeted an asteroid and began to attempt scanning it to find the maximum scannable distance. The mission computer informed me it was out of range up till 500m, then detected the ore. Next I targeted another asteroid, clicked scan, and the hud displayed that it was out of range. However the mission computer counted it. I was able to quickly scan all the nearby asteroids very quickly (I think up to 2000m). It seems like the mission is only checking to see if the asteroid is targeted, and not whether it is within scanning range.

hmm, ok.

I've also noticed odd behavior when taking the prospecting mission. If I targeted a roid when it was within 500m I instantly know what minerals were in it and the mission computer counted it (all without using the scanner).
If I targeted the roid when I was farther than 500m, the mineral readout said I was too far away. Then I would have to fly within 100m and use the scanner to get the info (or I could just target something else and retarget it once I was within 500m).

I was at mining level 0 at the time. Perhaps fleabait was at a higher level and therefore had a longer range?

Once you have an object targeted, it refreshes every 5 seconds. If you come within 500m right after it refreshes, you have to wait 5 seconds for it to refresh or change targets.