The recommend solr sharding see
will lead to a set of cores (depending how many years your statistic covers.
One for the current year: statistics
One for each passed year: statistics_[year]
The default statistic is set to display the last 6 month + the current month, but only uses the current statistics core.
So the total counts will be completely different to the priorly displayed data and only the month within the current core will have data.