Why are there three different lists of ownerless produce?

Question

The Mishna (Demay 1:1) gives a list of different types of produce exempt from demay. In Ma'asrot 5:8 and in Shevi'it 9:1, there are two more distinct lists of things that are exempt from ma'aser and which can be taken from anyone in the seventh year.

The reason they are exempt (according to the Bartenura in all three places) is because they are assumed to be ownerless.

These three lists are entirely different. What is the relation between them?

The most literal reading is that the first list is exempt from demay, the second and the third from both ma'aser and from not buying in the seventh year. This would be a good reason to distinguish the first case (not the second and third), but if so, what is the reason to distinguish between them?

It could also be that they're all exempt from all of these cases (demay, ma'aser, shevi'it), but if so, why are they given in three different lists? Elsewhere the Mishna repeats the exact same lists in different places, but here there are three lists apparently for the same reason.

The Rambam quotes the three lists separately (Terumot 2:12, Ma'aser 13:1, Shemitta Veyovel 8:12), after each one adding וכיוצא בהן, but with some differences in presentation which might or might not be significant. In Shemitta Veyovel 8:12-13 he brings the list from Shevi'it alongside the list from Ma'asrot, but I don't understand what, if any, contrast he is making.

Answer 1

One possible answer:

The Mishna Rishona to Shevi'it 7:1 says that the three cases are listed separately because they have different laws.

• The Mishna in Demay is dealing with fruits that grow by themselves. Therefore, they are exempt from demay even if it comes from someone's garden, unless he cares about them, in which case they would be obligated. (He tries to prove this from Pesachim 6b, but really the Tosefta Demay 1:1 also seems to back this interpretation.)
• The Mishna in Shevi'it is dealing with vegetables that can be sown, but are usually not kept, and are therefore assumed to be ownerless. But if you know they have an owner, they would be obligated in ma'aser.
• The Mishna in Ma'asrot identifies the different types with place names from outside of Israel (e.g. Egypt), and therefore they are assumed to have come from outside of Israel, so they are exempt whether or not they are ownerless.

For the difference between Shevi'it and Ma'asrot, I used the interpretation of the Rash (which the Mishna Rishona notes in his commentary to Ma'asrot). His explanation in his commentary to Shevi'it is that the Mishna in Ma'asrot is dealing with vegetables that come from specific places such as deserts, so they are assumed to be ownerless. Therefore, even if someone says he brought them from his field, he isn't believed, and they are exempt. I find this explanation less compelling, because I don't understand why you wouldn't believe someone who says he owns them. However, an advantage of this interpretation is that it doesn't contradict the commentaries (Rambam, Bartenura) who give the reason of being ownerless in all three cases.