eDiscovery

2012R2

I just created an in place discovery based on a keyword, I then exported it to a .pst.

I noticed today that some of the emails cannot be opened, "This item cannot be displayed in the reading Pane, Open item to read its content"

So I ran the same query but got the same results so I thought maybe those emails were encrypted so I delegated myself rights to the mailbox and now I can open those emails just fine, no encryption.

Any idea why the .pst file has random emails that will not open yet can be opened directly from the mailbox?