I m guessing its the thickness of the dot and dab then plaster board then tile adhesive then tile.
Gap between floor and wall tiles.
Place them between the tiles are you lay each one and grout straight over the top.
There is a gap of say 5mm to 8mm give or take high between where the floor tiles meet the wall tiles in the bathroom.
The d i y video how to seal a big gap between tile floor and tile wall in a bathroom or shower.
Not filling the gap can allow water to migrate undeneath the tiles.
If your gap is smaller than 1 8 inch in width use regular caulk.
For smaller tiles is advisable to use small spacers.
Never fill this gap with grout because tile grout is not flexible.
Fill gaps larger than 1 8 inch.
Caulking is the best choice for this application.
These can be left in place and will not be seen when you have completed your tiling.
Instead fill it with an elastic material capable of sustaining the movement of the floor and the movement of the wall.
It might loosen them and might rot the floor.
To read the rest of the thread look here.
As the tiles increase in size a larger gap becomes acceptable.
It shows how to seal a gap in the corner where the tile floor.
Gently push the tiles together so the spacers are tight in the joint.
The tile council of north america recommends at least a quarter inch gap between the tile floor and the walls.
The gap goes 3 5cm to 4cm back to the breeze block.
Gap between floor tiles and wall tiles.