I suspect that the difference is that Mapnik [1] does not render historic=castle (the first node), but does render tourism=hotel (the second node). If you switch to Osmarender (click the + sign on the top right, then select Osmarender) you will see the castle appear because Osmarender does render historic=castle.
[1] Mapnik is OSM’s default map renderer. Mapnik is not OSM, it is a separate piece of software which selects certain parts of the OSM database and renders them into a map which you can see on the OSM website.
If you’re going to be really picky, there’s also a piece of software involved called osm2pgsql which does part of the work in pulling out certain features.