I am not sure if it is officially declared by Microsoft or not, but SharePoint is set to delete/purge workflow history from database after 60 days. That means you can't find anything more than 60 days old in Workflow history list by default setting.
If you are using custom developed workflows that could be annoying because you'll have to keep approval workflow info vs. more than 60 days according to your company policies etc.
There is a simple way to prevent this behaviour. Just log on to Central Administration and Go to Timer Job Definitions on Operations page.
Find the "Workflow Auto Cleanup" timer job that belongs to the web application you want to preserve.
Disable it, thats it..