Good question, Tony!
This was all I was able to find. It could be how certain people are setting up the events on the calendar:
When people use the Outlook Calendar to schedule meetings and other events, they can see your availability unless you have changed the permissions for your Calendar to prevent this. Normally, appointments, meetings, and events have a Show As setting of Busy. You can also create items with a Show As setting of Out of Office. Items set to Out of Office are regarded as similar to items set to Busy — someone should not expect you to be available at that time. Visually, the two types differ — items set to Out of Office appear with a purple color, items set to Busy are identified by a blue color theme. This difference in appearance helps emphasize why you are not available. For example, you might be able to move a conflicting Busy item, but you're probably not able or willing to change an Out of Office item such as your vacation.