I believe that's normal - anytime a new account is added and calendar events are synced, it grabs the last month of events (and will grab older recurring events.) As you continue to use the phone since the factory reset. those events do not go away, so the back events will remain as you use the phone.

As far as I know, there is nothing that you can do to change that. Perhaps if you are rooted and back up calendar storage (then retsore after a reset) it would work, but even that I am unsure about.

If you need to search for older events you should be able to do that in the mobile browser interface to google calendar.

 

How often do you need to use calendar events older than 60 days?

 

See this post: http://androidforums.com/motorola-droid-bionic/639567-events-missing-calendar-ics.html#post5139453

By the way, the same fix works with Jelly Bean. I did a factory reset a couple months ago and had to go through the same process to get all 11 or 12 years of calendar data back on the phone.