knitting... nearly started!

I got my Rialto Aran and made tension squares, one on 5mm one on 5.5mm, do your squares turn out square? Mine is more of a rectangle! Unless i counted half purls as knit stitches? I made a garter stitch border, i.e. a few garter stitch rows to begin and end, and 3 stitches either side, leaving a 4inch stockinette stitch square in the centre, in theory, only mine is more like 4 inches wide and 3.5 inches high... any tips?