Mathematically, it is expected. With only 7 basic chords (C,D,E,F,G,A,B) to create a song, you will eventually reach all the possibilities even if you have variations, a different tempo and different words.
Almost all pop songs have a 4/4 tempo. This will translate into 4 chords at most for each lyric sentence. Of course you can have more or less but for a basic pop song, the number of chords for each sentence will vary between 1 and 4 chords.
Look around on the web for guitar chords and you'll be surprise how simple it is. For each lyric sentence, with 4 chords, you get a possibility of 2401 variations. But you can't just put random chords together to create a nice melody. Realistically, you may end up will only a hundred variation if you feel adventurous.
Last weekend, I was learning "All About That Bass" from Meghan Trainor on my guitar. Not really my kind of song but it's catchy and my daughters are crazy about it. I usually play classic rock and blues. Seeing me trying one of their favourite songs was a family event. :)
As I was learning the tune, I was having a hard time singing as I was always confusing with "What's Up" by 4 Non Blondes. Both songs do use the same basic chords (A, Bm, D/G). When you listen to both of them, you can clearly hear that they sound alike, "What's Up" being a bit slower.
We finally decided to create a mashup of both songs, just for fun.
Next time you hear the new hit on the pop chart and have a feeling of "Deja Vu", you now know why!