The mishna says that if one shechts a korban to eat the fetus in the womb out of the proper time, it doesn't create pigul. At the end the mishna says that if one creates pigul with the intent of eating the meat out of the proper time, the status of pigul wouldn't apply to the milk. There is a slight deviation in Rashi from the explanation as to why the fetus isn't me'fagel vs. why the milk isn't mefagel. On the fetus rashi writes - דלאו גופיה דזיבחא הוא, meaning that it isn't part of the actual body of the korban, but on the milk rashi writes - דלאו זיבחא הוא, meaning that it isn't a korban. What is the implication of this subtle difference?
The Cheishek Shlomo explains based on the Rambam (ma'achalos asuros 3:6) who says that milk of a non-kosher animal is forbidden by the Torah but there wouldn't be any malkus since the pasuk of malkus explicitly states "the meat" of the animal, and milk isn't meat. Similarly, the issur of eating pigul on an animal that has status of pigul is - ואם האכל יאכל מבשר זבח שלמיו, implying meat and not milk. Meaning, milk may very well be considered an inherent part of the animal, the violation of pigul is only for eating the meat and nothing else. However, in the creation of pigul one has to think to eat an inherent part of the korban out of the proper time, to the exclusion of something which isn't an inherent part of the korban. Therefore, when talking about the making of pigul rashi uses the term that the fetus isn't גופיה דזיבחא - it isn't an inherent part of the korban. But when speaking about a korban that is pigul and explaining why there is no kareis violation for eating the milk rashi says דלאו זיבחא הוא, meaning it isn't the meat of the korban. According to this approach it is possible that Rashi would hold that if one creates pigul by intending to eat the milk out of the proper time it would create pigul, but the cheishek shlomo rejects this possibility.
Perhaps rashi agrees that one cannot create pigul by thinking on the milk to eat it out of it's proper time. This was so obvious that the mishna never had to discuss this case because clearly the milk isn't an essential part of the animal - לאו גופיה דזיבחא הוא. But, one may have thought that on a pigul animal one would be in violation of eating pigul for eating the milk because although it isn't an essential part of the animal, it is still part of the animal. Therefore, rashi has to say - לאו זיבחא הוא that it isn't at all a part of the animal. However, the fetus on a live animal is more likely to be an inherent part of the animal to create pigul since it's existence is dependent on the mother, so it is necessary to speak out that pigul can't be created by intending to eat the fetus out of it's proper time. But once the animal is turned into pigul, the fetus is more of an independent entity and would not be part of the animal to be in violation of eating pigul. This is how rashi explains the mishna until R. Elazar in the gemara introduced the notion of not being mifagel to create pigul but being mispagel to be in violation of eating pigul. The gemara at the end conforms the mishna with R. Elazar. At that point rashi doesn't want to say that the fetus is enough part of the animal to be mispagel but not enough to be mifagel. Therefore, rashi offers another approach that the fetus is indeed part of the animal and should be pigul with the animal. However, there is a special requirement that to be mifagel it needs to be a part of the animal that is routinely eaten, to the exclusion of the fetus. In conclusion the fetus is part of the animal but not normally eaten so it is mispagel but not mifagel, whereas the milk is not mifagel or mispagel.