Fortune bug

Hello, i enchanted my diamond pixaxe . On pixaxe was fortune 3 i fuond diamond ore there was 5 of them.I mined diamond ore with my fortune 3 and its dropped only 5 diamonds. So i  create new world . I placed 10 of diamond ore and mined again with fortune 3 pixaxe and it droped 10 diamonds. Does fortune 3 not working ?