I think I know what their reasoning for it is:
They probably wanted to keep Iron Man happening in 2008, and keeping in mind the official timeline's 6 months between Iron Man and Iron Man 2 (with the latter movie taking place on the following calendar year) and a year between Fury's Big Week and The Avengers, means that The Avengers would be in 2010.
Feige has said that they're planning on making one Spider-Man movie for each year of high school (10th, 11th, and 12th grade). Homecoming 2 comes out 2 years after the first one, so if they continue coming out 2 years after each other, Homecoming 3 will come out in 2021.
If Homecoming 3 takes place in real time of the movie's release date, then that means Homecoming, since it marks the beginning of his 10th grade in high school, would take place in fall of 2018. I think the producers also came to that conclusion as well, since there were Stark Expo 2018 posters during set visits (which weren't in the film).
This disregards the implied placement of fall 2017 (due to Aaron Davis), but there have been prop contradictions before, so there will probably be other contradictions in the next movie.
Other things supporting this are Happy having the ring since 2008 (Iron Man's release date) and Cap being frozen for 65 years (1945-2010), therefore lining up exactly with 8 years between The Avengers and Homecoming (2010-2018).
They could have pushed the movies from Iron Man to The Avengers forward a few years, or pushed Peter's graduation back a few years, so as to not have any contradictions, but oh well.