Variations can be a little tricky! When a product has variations, Jungle Scout estimates its sales and revenue in two ways.
For those products whose variations have their own Best Seller Rank (BSR):
We estimate the item's sales independently of the other variations' sales. To see those variations and their individual estimates, you have to go directly to the product's listing.
The variations will be represented by an indented arrow to the left of each variation's title:
For those products whose variations all have the same BSR:
Because Amazon gives no indication of how those items sell differently from one another, we can't calculate the estimated sales and revenue for the variations.
In cases like this, we display the estimated sales for the product's entire listing (all variations combined) on each line:
The monthly sales are calculated differently as we consider the current BSR and historical daily sales from the last 30 days.
Sometimes, you will see different monthly estimates even though the variants share the same BSR. This is due to the different historical daily sales.
We calculate historical daily sales by capturing an average BSR on a given day and estimating the sales for that day.
Here's an example:
For variant A, the average BSR that we captured on February 16th was 8:
Based on that number, we estimate the daily sales for the entire listing would be 3605 (since all products share the same BSR). 
Now, for the second variant, we did not capture the average Best Seller Rank at the same time, and that will affect the historical daily sales, which will also 'skew' the monthly sales data - that is causing the difference in the monthly sales:
For variant B, on February 16th, the average Best Seller Rank was 7:
This results in 3796 units of sales for that day, for the entire listing: 
The numbers are close. Still, sometimes, we see higher fluctuations in the Best Seller Rank (due to how the product is performing on Amazon) and historical daily sales, depending on the changes in the Best Seller Rank throughout the day.