Contribution Calendar Boxes not showing all activity

Hi all,

For school I’m currently applying for internships and I noticed that not all my private contributions are shown on my profile if I look at it in incognito mode. Some days do and some days don’t.
When I hover over an empty box (grey) the tooltip says ‘no contributions’. However, if I click on it it actually shows all contributions in a list.

Why are not all boxes showing any color? I do have the ‘Include private contributions on my profile’ checkbox marked in my profile settings.