For some journeys I've found that there is an important amount of subscribers that clicked on email but didn't opened it. can you help find some explanation please? Thanks
You need to understand the difference between how clicks and opens are tracked.
Clicks are ALWAYS counted, as this is done through a redirect URL, so every click will result in a record in _Clicks data view. One can't simply avoid being tracked in a context of a click, unless the URL is implemented incorrectly (or deliberately in a way to avoid clicks being tracked)
Opens are being tracked through a tracking pixel, being a simple image (although the image has a unique URL for each email and recipient). Once the image loads, the open is being tracked. This is not a bullet proof solution, as a number of email clients block the tracking, e.g. in iOS 14.
This means that for the same recipient you might see a click being tracked, while no open has been registered. When doing a tracking extract in Automation Studio, you can ask to include inferred opens, so once an email has been clicked, it is also treated as being opened. However this only applies to the data in the extract, not in data views and Datorama reports.