One thing interesting about the ratings that are made public is that it is what you have to pay for is what really matters. For example, a station could be number 20 overall and appear to be doing poorly yet they might be one of the top stations in a certain demographic that is important to advertisers. For example, you might be #20 in 6+ numbers but #2 in males 35 to 44.
There is lots of truth in 'it's what they don't play'. Two similar sounding stations to the average listener might be reaching entirely different targets for listeners. Because music is selected based on how many in a certain demographic like a certain song, that song might also work well within a few age groups, male and female. When you look at a couple of stations and compare, the differences might seem minor, but it is how the music is arranged, presented and more importantly, what is not played by one of the two stations. At one point in top 40 radio perhaps 60% of the currents were shared by the adult contemporary format. In essence, it was simply the elimination of the harder edged songs and the delivery or image the station went for that really distinguished the stations between top 40 and adult contemporary.
I suspect among the four 'top 40' stations you can hear a distinctive difference if you paid close attention to a sample hour and how the station tried to image itself.
For example, I worked a market with three top 40 stations. One played more oldies in their mix, skewing older. One was straight top 40. We were rock leaning top 40. Even so, we all shared many of the same songs but the way we presented it was different. Ironically, we all did well and the station I worked for usually ended up on top since there was a big university in town and the lack of a rocker on the radio dial made our station the preference among rockers although they were quick to complain we were too tame for their tastes. We were simply in a market where a station going rock would be less successful than a top 40 leaning rock station and the result was the rockers listened anyway.