Are you positive that it's been grafted? What is the supposed scion variety? Is it a lemon? Where did you obtain the plant from? Did you try crushing a leaf and smelling to see if it leans toward any particular citrus?

it must be grafted, the rootstock and scion leaves are different

its supposed to be clementine

plant origin is from sicilia italy

here are more pictures

OK, so that does look grafted. Perhaps the rootstock is sour orange? Can you ask locally?