This is what we wrote in the Insights section which seems to answer your question:

(From Insights to Daf 73:3)

73b

3) DIFFERENT FORMS OF "ZOREI'A"

QUESTION: The Gemara concludes that "Zomer Chayav Mishum Notei'a" (pruning is Chayav because of planting), and "Notei'a, Markiv, and Mavrich Chayav Af Mishum Zorei'a" (even planting, grafting, and bending a vine branch and planting it are Chayav because of sowing). RASHI explains that since those three (Notei'a, Markiv, and Mavrich) are all forms of the Melachah of Zorei'a, if a person did all of them together with Zorei'a, one is Chayav to bring only one Chatas.

This implies that if one does Zomer, which is Chayav only because of Notei'a, together with Zorei'a, one would be Chayav to bring two Chata'os. But the Gemara just said that one who does all of these together is only Chayav one Chatas!

(a) The RITVA explains that according to Rashi, one is indeed Chayav two Chata'os, one for Zomer (which is Chayav because of Notei'a) and one for Zorei'a. But if he does Zomer and Zorei'a together with Notei'a as well, he will be Chayav to bring only one Chatas, because Zomer is part of Notei'a, while Notei'a is identical to Zorei'a (but done with a sapling instead of a seed). Since Notei'a and Zorei'a are covered by one Chatas, Zomer, which is a Toldah of Notei'a, is also covered by that Chatas.

(b) It is possible to learn differently in Rashi, though. When the Gemara says that Notei'a, Markiv, and Mavrich are Chayav "Af Mishum Zorei'a" (lit. "even because of Zorei'a"), it does not mean that they are Chayav for both Notei'a and Zorei'a. Rather, they are Chayav only for Zorei'a. "Af Mishhum Zorei'a" means that even all of these are Chayav only for Zorei'a (for we might have thought that they are their own independent Av Melachos and Chayav for both themselves and for Zorei'a -- see Background notes).

The Chidush of Zomer, on the other hand, is the exact opposite. It is obvious that Zomer is not its own separate Av Melachah. We might have thought that Zomer is not Chayav at all because it does not resemble any form of planting (since when one prunes a tree, he does not place any object into the ground in order to grow). Therefore Rav Ami is teaching us that Zomer is indeed Chayav for Notei'a.

(c) TOSFOS, quoted by the RITVA, disagrees with Rashi and says that the Gemara is discussing Hasra'ah (warning someone before he commits a sin), and not the obligation to bring a Korban or multiple Korbanos. The Gemara is teaching that one can only be Chayav for Zomer if he was warned with "Notei'a" (i.e. the witnesses warned him and said, "You will be Chayav if you do that act because it is an act of Notei'a!"). To be Chayav for Notei'a, Markiv, and Mavrich, though, one can be warned for either Notei'a or Zorei'a (but one will always be Chayav only one Korban, regardless of what he was warned for).