Example: a few months ago I had 10% share, but now it has increased to 12% for the same time period.
The model is constantly learning and adapting to provide estimates and insights with increasing accuracy; these learnings are applied retrospectively and so data from several months ago can be impacted. Rather than leave the old data untouched, we improve it based on our new learnings.
All historical data for the last 12 months is re-calculated monthly based on the most recent search volume data from Google.
Other reasons are that the search terms are continually being optimised and updated. We re-download the estimated search volume for each term frequently, so a new term with little impact on your account historically can become an important term if the number of people searching for it remains high.
To report data for a set period, i.e. Monthly and have a record of this that will not change, many users set up a Scheduled Report to run on the 1st of the month and can therefore be referred to month later if required.