为什么长音a;和双元音ei拼组合

［ai］ y_fly i_design igh_light ie_die
［596;i］ oy_boy oi_join
［ei］ay_day a_bake eigh_height
［au］ou_found ow_flower
［601;u］o_go oa_boat ow_show oe_toe
［i 601; ］ear_hear ere_here
［e 601; ］air_hair ere_where eir_their are_scare
［u 601; ］our_tour oor_poor