I always had my students cast on ten stitches. It gave me a


The usual reason for these extra stitches is that the yarn is not held to the rear of the knitting as a stitch is being formed. If you hold the yarn in front of the knitting as you make the first stitch of a row, that yarn has to wrap the needle to get in the correct spot for the first intended stitch. That accidental wrap becomes the first stitch. Thus the stitch count increases and a sloped edge result.
This most often happened when students started doing stockinette stitch. When starting a knit row, the yarn is held to the rear. When starting a purl row, the yarn is held to the front.