Hmm... maybe it's because when you are knitting flat you purl one row and then knit one row in order to get stockinette, but on the other hand, when you knit in the round you just keep doing only knits or purls in order to get stockinette. Get what I mean? when you knit flat, you are flipping the fabric from right side to wrong side, wrong side to right side, and so on, therefore you have to purl one row and knit one row. However, when you knit in the round you are just circling around the circle so you are not flipping the fabric or whatsoever so you just have to knit(or purl) for every stitch in order to get stockinette. This is one of the problems that I had when I first learned to knit in the round, I couldn't transform flat knitting into round knitting.
If you purl one row and knit one row when you knit in the round you will get garter stitch (which possibly is what you called regular knitting?), and when you knit flat it's the opposite. Get what I mean? (I know I'm not so good at explaining things...
Hope this helps!