Hi Perrin,
I believe that it is because in the present tense these verbs have an accent (unlike jeter , je jette ----> je jetterai) :
J'achète -----> J'achèterai
Je complète ----> Je complèterai
Je répète -----> je répèterai
etc.
Although you can also use an accute accent ( since 1990) it still feels very odd to me.
Hope this helps!