Non-full blocks occupy a full block when moved by piston

So I extended a piston with a rail in front of it, and while it was extending, the rail seemed to occupy a full block. The same thing happens when you retract a piston.

After some further testing I found that it happens to all non-full blocks. Fences for example, turn into wood blocks, carpet turns into wool and slabs turn into full blocks. (After the extension and retraction they are turned back to normal though, so it's only visual.)