Historical Monthly Organic Visit Data "Changes" Over Time

[hendersondavidp]

Hi Moz Community

I've got a Google Analytics question that I can't seem to find an answer for.

We have a client that we have tracked monthly organic visits over time.  From 2017- 2020.  At the end of each month we take the report from the previous month update direct & organic visits and other general observations, performance etc.

In December the client had an ask around validating the data because someone internally had been looking at Google Analytics and the total organic visits for the month in question was not the same as the data we'd entered in that month way back in October of 2017.

In November 2017 when we pulled the organic data we showed 485,891 organic visits.  In Dec 2019 we pulled the same organic data and the total was now 485,234 organic visits.  July 2018 451,589 when pulled again in Dec 2019 that organic number was 446,156.  A few hundred here or there isn't a gigantic deal but in some cases were talking about a difference of thousands of organic visits.

Spot checking various months from 2017 through to Jan 2020 shows these monthly organic totals having some pretty big swings against the number originally pulled.

I've never seen this and I'm not sure how I should be explaining to the client why our old numbers don't line up against the same data pulled more recently.