Hey, Andrea
Thank you for reaching out. Placeholder Density is a function of Display Ad Count divided by Max Ads Count (max ad percentage), weighted by Page View Count for a range of around 4 days. So, it's not just a case of having placeholders on the page it is also a case of those placeholders being interacted with.
Usually, the issue you are facing, means that there are placeholders that exist but that have been excluded on the top pages or something like that.
Have you excluded any placeholders or pages on tablet?