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!