Organic Traffic is an estimated monthly figure based on the past 30 days. 

We calculate the organic traffic of each keyword at the respective ranking position of the target website/URL by multiplying the estimated search volume of the keyword and the unique CTR of the target website/URL's position in a SERP.  

The key thing to note is that not all target URL/domain's position in a SERP is updated at the same time. 

We update the target URL/domain's position in a SERP, once in a few days to once in 1-2 months depending on the popularity of the respective keyword. 

So in other words, a more popular keyword gets their SERP updated more frequently than a relatively less popular keyword. For example, a popular keyword that is updated once in a few days will get reasonably well estimated monthly traffic However, a less popular keyword that is updated once in 2 months will have its monthly traffic estimated from the last time it was updated, which may be a little longer than the previous 30 days. 

Thus, Organic Traffic is really our best estimated monthly figure based on the past 30 days, bearing such nuances in the frequency of update of certain less popular keywords.