Why do maps that use OSM as a backend have more detail that OSM?

Where is it coming from? Are there other open sources of map data?

I've seen multiple mapping websites that provide business solutions that seemed to be sourcing their data from OSM. However, when I zoom in to known problem areas, I find a lot more detail. They're not getting it from Google, Bing, or Apple, and I find it implausible that these small specialized companies are sinking tens of thousands of hours into adding fine map detail. So where are they getting it? If it is open source is there a way to merge it back into OSM?

For example this website, maptiler, cites open street map as their source. Compare it with the official site. I have found multiple examples of the same thing. Can anyone explain to me what's going on here?