In the creation of Homunclei (sp?), the failed hunan transmutaion takes a part of the alchemists body as equivilant trade. The resulting creature remains in this world. Once fed Red Stone, or incomplete or imperfect Philospher's Stone, they regain their original forms and therefore become Homunclei.
However, Wrath does not follow this method of "creation". We can first assume that Izumi Curtis had and lost her baby a few years before meeting the Elric brothers. We can assume this because the boys are 10 and 11 when meeting her and by then she has perfected the use of alchemy with the use of an array. Therefore we can assume that she had her child as least one year before meeting the boys.
When Izumi performs her human transmutation, she diverges from the path of making a homunclei. She, having returned the creature to the Gate, did not bring the creature into the world. This left the creature on the other side of the gate, unable to leave.
Once the Elric's performed their transmutation, the creature, seen as the only being in the gate that seemed to have a body, was able to obtain Edward's right arm and left leg. This then allows the creature to perform alchemy.
Wrath, now able to perform alchemy, was able to escape the Gate to enter into the world. Having emerged from the Gate as a product of human transmutation, he bears the mark of the Orouboros, which is the sign of renewal of life or regeneration of life. However having never eaten the red stones, he is not a homunclius until Envy gives him a taste of the stones.
if there are typos, i blame new laptop wth crazy keys