A click on the Visibility Score change of any keyword group reveals a detailed explanation of what influenced the Visibility on that list of keywords, in the selected time frame.
Besides the rank changes that impact the Visibility positively or negatively, there is another factor that affects it, namely search trends:
Search trends and Visibility
Search trends are changes in the average monthly search volume of keywords. The Visibility Score formula takes keyword ranks into account, weighted against their search volumes. It is therefore not surprising to see search trends influencing Visibility.
The Visibility Score represents an impression share on a list of keywords: out of the total number of Google searches on those keywords, how often was your website viewed by a user among search results? It thus depends on ranks, which give the impression rate, but also on search volumes, which give the total number of searches.
Visibility increases when rankings improve, but this can also happen when keywords for which you already rank well would gain in search volume or when keywords you don't rank for would lose search volume. But the opposite can also be true: you can lose impressions without losing ranks, in case the keywords for which you have good ranks aren't searched for anymore.
The "iphone" example
To illustrate this, imagine having a list composed of two keywords: "iphone 7" and "iphone 8". In September 2016, you ranked very well for "iphone 7", which has millions of searches every month, but you didn't rank for "iphone 8", which didn't exist yet, so it had a negligible number of searches.
Fast-forward one year and we are in September 2017. Nothing has changed: your website still ranks very well for "iphone 7", but not for "iphone 8". However, "iphone 8" gains more and more in search volume every month, to the disadvantage of "iphone 7". The total search volume for the two keywords hasn't changed, but the distribution of searches between the two has. Your website will thus be less and less visible for this keyword list, in spite of maintaining its ranks.
An important insight
Your tracked keywords change search volumes all the time and this affects the impression share, so it is necessary that the Visibility Score reflect this as well.
We already knew that SEO performance meant ranking well on the big keywords, those with high search volumes. The valuable insight is that it is also important to keep the search trends in mind, when planning the SEO strategy, so that you prioritize the keywords that gain search momentum.
In SEOmonitor, you can evaluate this trend by using the Year-over-Year metric. In comparing keywords' current monthly search volumes with the figures from the same month last year, the Year-over-Year percentage serves as a relevant indicator, precisely for the search trends within your keyword portfolio.