Why do hairline cracks develop with terracota tiles?

I've noticed that hairline cracks develop between my exterior terracota tiles and the cement grouting. Is this because the tiles and grouting have different expansion/contraction characteristics under changing temperatures? Or is it due to inadequate stability of the CFC sheeting on which they are laid? (It's really annoying that the tiled balcony regularly leaks into the garage underneath. I've tried pushing silicone and sikaflex into the hairline cracks, but this only fixes the problem for a few months.)

I've had terracotta tiles on my front porch for about 15 yrs, laid on concrete. No sign of any cracks, I'm guessing that in your case the surface they are laid on may have a small amount of movement.

if the balcony leaks its not because of the hairline cracks im afraid, you see a waterproofing membrane would stop any water traveling to the structure underneath. most tiles will absorb water/moisture especially terracotta, unless of course they are porcelain/vitrified, and even then the grout joint will allow the moisture into the sub floor.. so no amount of silicone will stop the floor from leaking ..

the correct way to repair the floor and solve your leak would be to rip the floor up, apply a waterproof membrane and then retile the balcony.