Agenda vs assignation

Dear all,
I don't understand why one of my project is planned over the validated limit.
Indeed, I have enough resources to realize the project at time, but if I look at the planning, the project is in red, and the end date is 2 weeks after the validated limit. It is not possible to act a such end date because my client can't wait 2 weeks more.
I don't know where the pb is.
One explanation is ... I don't understand with precision the notion of "rate", when I affect a resource to a project.
I have seen that if I check the resource, i could see that all my resources are affected over 100%... but if Redaction is 100%, and Journal Issue 144 is a subproject of Redaction, why the rate are cumulated?
Other explanation is... I noticed that the Agenda don't respect my assignation. For example, I have assigned 30 hours to Eric to "write the monthly articles for the webiste"... but if I look at his own agenda, and if I calculate all the planned items with "write the monthly articles for the webiste", the whole time planned in this Agenda for this activity is really incredible. More that twice what I have planned. Why?

If someone has few time, I could create a temporary access for someone.
All the best,
Christophe
PS : Je peux aussi écrire en français, je ne sais pas s'il y a un forum spécifique pour les francophones.