Don't assume that when you shoot a round more than 100 points below your current rating it will be dropped for the next update. Your next rating may have dropped enough from other rounds that the 800 is not more than 100 points lower. No round is permanently dropped except DNFs. If your rating drops, a previously dropped round may come back into play.
The unofficial ratings process does not use the same calculations as the official process primarily because the unofficial ratings only look at the current round of scores and the official process uses all of the scores shot on the same layout if the weather is similar. It's also unofficial because sometimes the TDs will not set up the course layouts properly online to produce the appropriate ratings even though most TDs get the course layouts done correctly in their Excel tournament report sent to PDGA.
Rater of the tossed arc.