Google has informed us that they have updated how they measure the metrics they report in Google Search Console’s Search Analytics report specifically for search results in lower positions. Google updated the data anomalies page to say that this change started on July 14, 2017, and goes forward from there.
An incremental improvement in Google’s logging system now provides better accounting for results in lower positions. This change might cause increase in impressions, but also a decrease in average positions. This change only effects [sic] Search Console reporting, not your actual performance on Google Search.
This came up yesterday when we reported that many webmasters were noticing changes in the average position metric starting after July 13.
This is not a bug, as we previously thought; it is, however, a feature change in how Google measures the data in the lower positions.
Google is clear to say that no actual ranking changes have occurred specifically around this report, but rather it is how Google accounts for those positions in the Search Analytics report.
Source: Search Engine Land