See here for one official explanation there could be others. http://support.microsoft.com/kb/2649499?wa=wsignin1.0
Possible workarounds for deletions only.
1 - Go offline first. The update will go into the Outbox. Delete the update from the Outbox. You then ask the people you deleted to remove the meeting from their calendars.
http://www.slipstick.com/outlook/calendar/change-outlook-meetings-dont-send-updates/
http://www.msoutlook.info/question/582
2 - You can try unticking in the Scheduling Assistant rather than deleting names.
Possible workaround when adding.
I no longer have to use this, so I may not remember the steps correctly. Save without sending. Try to close then send the update.