Tum'ah Yotz'ah Alav mi'Gufo means that the person is the source of the Tum'ah. He is not contracting Tum'ah from an outside source that transferred its Tum'ah to him through touch (e.g. touching a Sheretz, Neveilah, or a dead body).

It seems that it is logical that when the Tum'ah comes from inside the person himself and is not merely passed to him from the outside, it is a higher level of Tum'ah, and therefore should have more severe Halachos.

For the sake of illustration, one could suggest a comparison from the Halachos of Kashrus. Eating non-kosher meat itself is considered a more severe transgression than eating kosher meat which was cooked together with non-kosher meat -- even if the kosher meat acquired a strong taste of non-kosher meat through the cooking process. Similarly "Tum'ah Yotz'ah Alav mi'Gufo" means that there is primary Tum'ah which comes from the body itself. If somebody merely touched a corpse, he only has a secondary Tum'ah.

D. BLOOM