Ziz said:
The star destroyer launched a half dozen probes down onto the planet. Why can't the one we saw be one of them and the one Luke saw be a different one? A third could have been the one that saw the shield generator and the one that Han and Chewie shot could have been a fourth.

It doesn't matter which probe droid is which.

I always got the impression that only 1 probe landed on Hoth. That's all we see.

One could even argue that what Luke saw crash is actually a meteorite, and not the probe droid. The movie is ambiguous (probably intentionally) about that.

Aside from Force visions, the timeline always moves forward in the Star Wars movies. So having the probe droid crash, then moving back in time so that Luke sees it crash, just doesn't feel right to me.