Suppose we get error reports from a machine running Ubuntu version N.
Later, we get error reports from the same machine running Ubuntu version N+1, N+2, or whatever.
Provided that we don't get further reports from it running Ubuntu version N (indicating a dual boot), it's safe to assume that it doesn't have N installed any more.
However, we don't remove it from the count of machines running N until 90 days has passed without a report.
Suppose we get error reports from a machine running Ubuntu version N.
Later, we get error reports from the same machine running Ubuntu version N+1, N+2, or whatever.
Provided that we don't get further reports from it running Ubuntu version N (indicating a dual boot), it's safe to assume that it doesn't have N installed any more.
However, we don't remove it from the count of machines running N until 90 days has passed without a report.
Hypothesis: If we fix this, the October-to-January dip in <https:/ /errors. ubuntu. com/?release= Ubuntu% 2012.04& period= day> will disappear, because it was caused by 12.04 users upgrading to 12.10.
Similarly, the post-April plummet in <https:/ /errors. ubuntu. com/?release= Ubuntu% 2012.10& period= day> will disappear, because it was caused by 12.10 users upgrading to 13.04.