Calendar shows lighter/greyed out items

Question

Here's a screenshot of my Windows 8.1 Calendar.

As you can see, some of the items appear lighter/greyed out (ie on the 15th, 21st).

Why is this? I have not noticed any difference between these items, and none of the ideas I've had for why (ie items without a specific time are lighter) have fitted.

My calendar is synced to hotmail.

The calendar online (hotmail.co.uk). (There are a couple of new events, but nothing should have changed with the earlier ones.)

This event appears lighter.

But this event does not.

Answer 1

Per the comments section, posting screenshots of the event details in question show what the differences are: both are All Day events whereas the others in the calendar are not, hence the difference in color.