I wonder... Would it be too far-fetched to think that Sesame Workshop figured out their mistake but since they had cited season 1 as the first appearance so many times that they chose to keep it that way? Of course I shouldn't start rumors or conspiracies or anything like that.
Then again, some of the clips in the 40th anniversary DVD are presented as beign from different seasons than what was in the timeline in The Street we Live On (and all the same clips up until 2003 were presented in the timeline in the 40th anniversary book bonus DVD). The timeline incorrectly lists season 10 as the year of "One You Go Beep" while the DVD lists the correct season 11. The timeline lists Teeny Little Super Guy as from 1982 but the DVd has one listed as season 15 (and I've seen a press release that cited season 15 as the debut). I don't know what to say about the Luxo Jr skits. The timeline says 1990 for one, and the DVD lists 1994 for another, but they very well could have been made in different years. In fact I think I read something about Pixar making one short for Sesame Street every few seasons, instead of all in one year.
And should we count it as a mistake that the DVD lists the celebrity version of Put Down the Duckei as season 18 instead of 19? Unless maybe the non-celebrity version really is from season 17 (the song is copyrighted 1986, and many songs seem to be copyrighted the year that the seasons end) and it's representation of that season in timelines are referring to the celebrity versions (despite not showing any portions with the celebrities). But then again, we do know the celebrity verses were recorded when celebrities were on teh show, and we do know that some of those celebrities were on teh show in season 19, in same sets and attire.