Effective effort must be different from % completed


I have a question regarding the « completed » field.
It seems that it is calculated from the total effort and the effective effort. For exemple for a task supposed to need 100h, if I say that 50h have been used, then the task is suppose to be completed at 50% and this is what I see in the « completed » field.
BUT, sometimes, after 50h the task is done at 60% or even 100%. How can I deconnect this automatic calculation ?

Thank you !