It looks like Musopen, at least partially, created their sheet music archive by scraping IMSLP.

I picked a somewhat well-known (but not too well-known) composer (Wilhelm Stenhammar) and selected three works at random. In each case, Musopen's file is the same as the topmost file on IMSLP.

https://musopen.org/music/22022-violin-sonata-op19/

https://musopen.org/music/22020-symphony-no2-op34/

https://musopen.org/music/22021-turandot-op42/

https://imslp.org/wiki/Violin_Sonata%2C_Op.19_(Stenhammar%2C...

https://imslp.org/wiki/Symphony_No.2%2C_Op.34_(Stenhammar%2C...

https://imslp.org/wiki/Turandot%2C_Op.42_(Stenhammar%2C_Wilh...

Far, far greater selection. Also, within a given work, there are multiple scores published at different times, so you can see the editing from multiple perspectives. For example, different fingering suggestions on piano pieces, etc.