Grout question

Hi, I'm currently tearing down a shower for some repairs, and one of those seemingly obviously stupid questions came to mind...

When tiling a shower wall with 10mm thick tile (I'm assuming the total depth ends up around 12mm with adhesive bed?) & ~2.5mm grout lines, should the grout end up being solidly packed into that entire depth? Or does the hydraulic resistance so to speak sometimes prevent this?

I ask this, as the current grout has failed, and upon removal I found I was just able to poke holes with the grout saw in places, as the grout only seemed 2-3mm deep - which obviously seems wrong, but I was unsure as to the exact proper method/outcome?

Thanks!