There are two types of cement based grout, sanded and unsanded. The sanded is use in grout joints that are wider than 1/8", unsanded is used when the joint is smaller. On floors unsanded is most commonly seen with rectified stone tiles, maybe this is what you saw.

All of the sanded grout joints can be smoothed with a sponge during the installation and clean-up of the grout, but nothing will remove the coarse look of the sanded grout. With most tile the grout is not even with the top of the tile but rather lies a little below the top of the surface. Sealing the grout wont stop dirt from getting intot he crevices of the coarser sanded grout, its purpose is to slow down the absorption of liquids into the body of the grout, dont be fooled in thinking that sealer is like a coat of polyurethane that puts a blanket over the grout. Sealer is comprised of resins suspended in a liquid. When applied the liquid evaporates and the resins fill the small micropores in the grout surface.