I know that it needs to be raised a little bit so that the floor can slope towards the drain. But on my floor the slant towards the drain is shallow, but the floor of the bathroom is much higher (3cm/1.2in above tiles) than compared to the hallway adjoining it. It is a concrete subfloor.
I would like to re-tile the bathroom, and I would like to lower the floor... but is there a special reason why it is how it is? Am I making a mistake changing it?
It's in an apartment - all the walls and floors are concrete. There is no wood at all anywhere.
In some older homes built between 1900-1930's it was common to set the ceramic tile in a bed of cement. The cement was poured over the subfloor. While this was a great stable base for the tile it did raise the floor. You may also have a case where the floor was damaged and during the repair or remodel they added additional layers of plywood or tilebacker to form a base for the new tile.
Because a lot of times when people remodel they are lazy and just cover up what was there. There could be layers of plywood, there could be concrete (fun), roofing shingles, backer board... I have seen people mortar/concrete over tile and lay tile. Yes you can remove it all. Worst case scenario is that you have replace a rotting subfloor with plywood. Make sure that you have a little access below this room if you are going to be demoing it all - you will probably have to cut into the ceiling below for access. At least plan on it. (See why people are lazy)
Because the bathroom tile sits on top of layers of other materials. Beneath the tile is a layer of thinset mortar, and beneath that is a concrete backer-board which is either 1/4" or 1/2" thick. All of these layers sit on top of the original subfloor, which makes the bathroom floor higher than the rest of the house, unfortunately.
I have removed transitions at the doorway and looked in from the side. You can see the layers of what is there. If you see particle board in the hallway or some other material not related to the build date, there may be an additional 1/2 to 1 inch in the bathroom area that will not be visible.
Floor joists in some old houses are spaced at 24 O.C.. Tiling over a single layer of plywood/OSB underlayment over such joists is not to code any more, and it requires a second layer of plywood. Because of that some contractors used to add second layer of plywood which resulted in higher floor levels in bathrooms.