After giving the web page a transitional ranking, Google then monitors the behavior during the transitional ranking period for an undetermined amount of time. The random transitional ranking period is effectively an opportunity for Google to investigate things further and “test” the owner of the website to see how they respond to ranking drops or fluctuations.
Someone who keeps changing their website in response to changes in ranking is probably using techniques that Google does not approve of, and if that’s the case, Google will penalize the page or site in question.
For an example, let’s say that someone decides to stuff their web page full of keywords. If this action results in the page dropping in rankings because of Google's transitional rankings, the person responsible for the spamming might then respond by completely removing the keyword stuffing from the page.
In the same way, if Google keeps the ranking of the page the same as it was before the keyword stuffing was implemented, the spammer might be disappointed that the technique didn’t positively affect the ranking and therefore respond by adding in more spamming techniques to the page in a bid to raise the ranking.
Both of these actions would send a red flag to Google that illegitimate ranking modifying techniques are being used, which may then be penalized by Google in the final ranking – which it calls the “target rank.”
On the other hand, the likely response to a rankings drop by someone using legitimate techniques would probably be that they’d do nothing, in which case, Google would (after an undetermined period of time) restore the web page to it's original ranking or most likely give the web page a higher final rankings then where it was before the transitional period.