Why are unopened meeting requests not on my calendar as tentative.

Meeting requests are not reliably and timely showing up on all requested
invitees calendars, sometimes will show up as tentatively almost immediately
and sometimes takes hours or even days or may never appear. This only
happens if the invitee has not yet opened the meeting request, they will show
up once the user opens the meeting request and responds, however if someone
is out of the office or for some other reason has not yet opened their
invite, the meeting request will not show up on the calendar as tentative, so
will appear unreliably as "free" time on their calendar causing potential
meeting conflicts. Seems to be more common for those using OWA.