It is important to keep in mind that the sales rank changes very often. So, if you scan a product, it'll show the sales rank at the time of the scan. If you check the sales rank on the Amazon listing a bit later, it could be different if it just updated.
You always want to visit the listing on Amazon and see what the sales rank is directly on the listing. You also want to check the Keepa graph to see the history of the sales rank, and what the 90-day average is.
In this first picture, we see on April 5th the sales rank was 176,616.
In the second picture, you can see on the same day, the sales rank is 65,428.
So, this shows that the sales rank can vary quite a bit. This is why we like to look at the 90 day average on Keepa.
In the third screenshot, you can see how the 90 day average takes the average of the sales rank.
Whenever you see the sales rank graph is consistently up and down like this, as if you could draw a straight line through the middle, it is a great time to use the 90 day average.
A bad example of the 90 day average is if the sales rank was very high for the first month, and then very low the next two months. Chances are, the sales rank would stay lower, so the higher rank history could skew the results.
Should I use my analyzer tool's sales rank or Keepa's?
What is the most accurate BSR (sales rank)?