The way that it works is that there are only 3 slots of top ads shown at one time.
The site arranges the ads by category and location.
So when there are several ads that have been upgraded to 'Top Ad', which there usually is, the site randomly displays a different 3 each time that you navigate to the next page, or if you were to refresh the browser.
Now as someone refines there search, your ad will naturally have less ads to compete with.
Example; Your ads fits the following criteria, and assuming that you've filled out all of these fields in your ad: (Location = Ontario), (Category = Firearm), (Sub-Category = Rifle), (Action = Lever), (Manufacturer = Winchester), (Caliber = 30-30)
If someone searches by (Category = Firearm) or by (Location = Ontario), your ad will be found somewhere in the top ads, but again will be randomly selected for 'Top Ad' between the other competing 'Top Ad's that fit the same search criteria.
But for this example, we'll say someone searched by (Category = Firearm). Let's say your ad is competing with another 12 ads that are (Category = Firearm) and currently using the 'Top Ad' function. Your ad may be displayed in those top 3 slots on Page #1, or it might not be displayed in the top 3 slots until Page #4, it's randomly choosing between 12 ads at this point.
But, if someone chooses to refine, or narrow, their search further to (Location = Ontario). Now you might only have 7 ads that are competing for the 3 'Top Ad' slots.
If someone refines their search even further and narrows to (Action = Lever). Let's say in this case that you and only 2 other people who is using the 'Top Ad' function that fit this criteria. Since there are only 3 'Top Ad's that fit this criteria, all 3 ads will obviously fit into the 3 available 'Top Ad' slots. Even if there are 5 full pages of guns matching all of this criteria, your ad will be displayed in the 3 'Top Ad slots on all 5 Pages automatically since there are 3 ads or less to using the Top Ad function matching this search criteria.
As where if someone searched just (Category = Firearm), there could quite a few others with (Category = Firearm) using 'Top Ad' at the moment for the site to randomly display 3 of the ads promoted to top. You might search firearms and see yours right a the top on the first page, or it might be held at the top on the 2nd or 3rd page. Each time you visit the site, it will randomly display them in a different order.