This^^
Basically the aiming point is calculated as if the Bomb continues down on its ballistic trajectory without dispensing sub-munitions. Which of course is not what happens... the bomb sub munitions are dispenses far off the ground (and back behind the aiming reticle) and then drop directly down.
Current solution? Aim behind targets + takes practice or... go for a very steep, nose down, dive bomb, so that the reticle is close to the point under which the BLUs deploy.