That was very interesting!
Each top 20 set included lots of similar sounding songs. And that is what I'd expect since the top 20 is driven by market demand...what sells, what is popular is what gets into the top 20. The top 20 (or 10 or 100 or whatever) has never been an artistic measurement.
With that said, this is a very subjective comparison and I bet there are plenty of days where the top 20 list has more variety. So, while interesting, I don't think it is very meaningful.
I'd love to see this same sort of top 20 comparison for every decade and maybe take the top 20 songs of the year or of the entire decade instead of a single day snapshot! I bet lots of 80s top 20 songs were similar. Same for the 70s and 60s.
And, of course, I call BS on the tired old saw that today's music is somehow not real music. I heard that same complaint from my grandpa and then from my dad and now I hear it from my peers. And also we had to walk to school, through the snow, with no shoes and it was uphill both ways!

Now, get off my lawn!