Among the cards displayed in the time line, there is one that does not have as a green check. What is the difference?

Card check is displayed when they are "cleared"

Clearing conditions are different, depending on the type of card.

If the card is a study one, and does not contain a quiz, just read it will clear it.

In the case of a quiz card, it will be cleared when the good answer will be given. However, depending on the edition's settings and decisions of the editor, it can be configured that the card is cleared just by looking at it.