I have the same sort of problem..........But with a twist!
Look and see if you have it too. Do your syncing, back and forth as normal until you get it right, i.e. both outlook and BB calendars are the same.
Unplug your BB and don't sync again for the rest of the day. Next morning before you sync, check to see if your BB calendar matches your Outlook Calendar, I bet it doesn't.
In my case, the DM tries to delete about 500 Outlook entries each time I sync, no matter what settings I put into the DM (either two sync all items, or by syncing 999 days before and after), it just wants to delete them. When I tell it to re-sync, it gets it right, but stuffs up the few changes I've made to the BB in the mean time prior to syncing.
I thought the stuff ups were happening at sync time via USB, but I found that the missing endtries on the BB, were what was causing the DM to try to delete the Outlook entries!!!! The BB is somehow losing data some time during the day, causing the DM to try to delete the outlook data, its not accutally at sync time that the prob exists.
Very strange, it looks like the BB device is trying to only hang onto 3 months worth of data. Just checked the cal to see where there is no more entries and its three months.