That is because the three stations mentioned beforehand have Hot AC's as their sisters under the same company. Hot AC's tend to skew younger while mainstream AC shifts older. So when one company owns both Hot AC and Mainstream AC, you will be hearing much older stuff on a Mainstream AC than on a Hot AC, some dating as back as the early 70's.
I agree. This is also true for the St. Louis market with AC KEZK and its sister Hot AC KYKY and the LA market with AC KOST and its sister Hot AC KBIG.
There's also an opposite effect when a Mainstream AC have a Classic Hits as their sister. The Mainstream AC tend to skew younger while the Classic Hits tend to shifts older. So, when one company owns both a Classic Hits and a Mainstream AC, the Mainstream AC tends to lean less on older music, less on 80s and 90s, and more on recurrents and currents.
A strong example of this is the Dallas-Ft. Worth market. AC KVIL, Classic Hits KLUV, and Adult Hits KJKK are all owned by CBS. KVIL tried to skew young over the early part of this decade and it was deemed unsuccessful. They retooled their format in 2013 and dropped 80s altogether and adopted a new slogan as "Best Variety 90s, 2k, and today", resembling CBS Fresh format. In 2014, KVIL was placed on the Hot AC add board according to Mediabase. Another possibility to the demise of the AC format on KVIL is that Dallas-Ft. Worth is in the Bible Belt. Unlike most markets around the nation, DFW has one of the most successful Christian Contemporary Music station in PPM 6+ A.K.A. KLTY. CCM can be a very competitive format against AC.
There are a few exceptions to this in certain markets.
When comparing the Oklahoma City market to the Houston market. In Oklahoma City, both AC KMGL and Classic Hits KOMA are owned by Tyler Media and in Houston, AC KODA is owned by iHM. Houston doesn't have a true Classic Hits, it's rather a Classic Hits and Classic Rock hybrid A.K.A. KGLK owned by Cox Radio. KMGL still plays a handful amount of 80s (3-4 80s an hour) and KODA also plays a handful amount of 80s (3-4 80s an hour). KMGL has a strong Classic Hits sister, but KODA doesn't have strong Classic Hits as its sister, nor the Houston market has a Classic Hits at all.
When comparing the New York market to the Houston market. In New York, AC WLTW is owned by iHM, Classic Hits WCBS-FM is owned by CBS, and Hot AC WPLJ is owned by Cumulus, much like Houston, except New York does have a true Classic Hits. WLTW still plays a lot of 80s (5-6 80s an hour), very few 70s (1 70s every 2-3 hours), few 90s (0-2 90s an hour), while KODA plays a handful of 80s (3-4 80s an hour), some 90s (1-3 90s an hour), and mostly recurrents and currents. I thought KODA could leaned a little bit older to fill the missing gap for a Classic Hits. KODA should at least play more 80s tracks like WLTW.
In Sacramento, CA, both AC KYMX and Hot AC KZZO are owned by CBS. Even KYMX has a Hot AC as its sister, it still skew younger. KYMX only plays 2-3 80s an hour which is less than KMGL and KODA. Even KMGL has a true and strong Classic Hits as its sister, it still play at least 3-4 80s an hour.
In Miami-Ft. Lauderdale, both AC WLYF and Classic Hits WMXJ are owned by Entercom. Even WLYF has a true Classic Hits as its sister, it still skew older with a lot of the slower tracks from the 70s, 80s, and 90s, with a few recurrents and currents. It plays about 1 70s every 4-5 hour, 3-6 80s an hour, 2-4 90s an hour.
There are other factors such as the demo in the market and how the PD decided to program their AC.