I think that the 2 narratives in Mark 4 : 26 and Mat. 13 : 36 are referring to the same parable about the wheat and the weeds. In both gospels, the account is situated between the parable about the sowing in the rocky soil, etc. and the parable about the mustard grain.
Part of de discrepancies can be explained by the fact that Mark's narrative is more concise. For example, it says that the sower sleeps, when Matthew reads "while the men were sleeping". You may remember that the society teaches that Solomon building the temple means in fact that he got the temple built by his workers. The same reasoning could be used here but the WTS just forgets to apply it because differenciating the 2 accounts suits their purpose, which is assimilation of the sowers in the first and the second parable in Mark's, and only Mark's account, the one where we don't read that the sower is the son of man, and leaving aside Matthew's parallel account. dishonest approach, we'll all agree.
Now, trying to answer Bonnie's question, we notice that in 21 and 28 sept study articles, they apply right now events which the Bible says will occur at the conclusion of the system of things, namely, harvesting in the wheat/weeds parable (which they say represents those who study with the JWs coming to chistian maturity) and the sorting out of fish, which they say is operated now, whereas Matthew says that it is performed by angels who throw the wicked in the fiery furnace.
My assumption is that the WTS holds that view for the reason that they want to get JWs accustomed to the idea that Armaggedon is not for tomorrow morning and not even for the day after tomorrow, and the society gradually adopts a long term perpective which will justify her being still around in 50 years from now, as a mainstream religion. In addition, this stand allows R&F JWs to have the satisfaction of seing right here and now the result of their preaching work : harvesting and selection of fish. Did anyone say manipulation ?