MySql Databe question

Hello all,

We use Piwik for 2 months now to measure the audience of a single website for now, and we are very satisfied. It’s a great product, congratulations to the whole team for the work done.

Viewing the number of daily visits, I have set up the archiving of reports by CRON as advocated. But I see tables in the MySql database that I do not understand.
For example, Piwik_archive_blob_XXX, piwik_archive_numeric_XXXX tables for months 2008, 2009, or 2012. Piwik was not installed on those dates … Most of these tables are empty, but some contain data that does not match nothing.

Better (or worse), I saw this morning a table for the month of February 2017, containing data, while we are January 31.

Why are these tables created ?