When you knit in the round, you are always knitting the same side of the project.
To stockinette in the round, you knit all rows. For garter stitch, you knit a round, purl a round.
Flat knitting is the opposite. To stockinette, you knit a row, then purl a row. Garter stitch is knit all rows.
Think about if you were to knit back and forth on two needles, so you never turned the work. It would be similar how knitting in the round works, because you always have the work in the same direction. Thus, all you knit stitches and purl bumps end up on the same side every time.