Default Log Retention
You should take a look at the raw log data. If you have an ES, its in the E:\nic\lsnode\data folder. On an LS stack, it on the NAS, in a similar path (on each CIFS volume).
Since the data is stored with the date in the folder, you can find the oldest data present. (try searching for files named 2010) That doesn't mean it is indexed (which is a separate task to re-index).
Assuming everything has been working fine, I'm guesing there is only 10 months of data, and some task was purging data. Otherwise, it could be a data corruption/loss issue.
Regarding tasks, check the Windows scheduled tasks (on each envision server), AND the enVision Scheduler Service tasks (again on each envision server). Either is where you would run the necessary command(s) for data purge.