The main idea about Japanese letters (Kanjis, hiraganas and katakanas) is that they are mainly written using a paint-brush and that they abide a certain stroke order. so the letter Sa is written as three strokes: stroke one (the upper dash), stroke 2 (the line from up to down) and finally stroke 3 (the lower diagonal line).
Therefore the Sa letter should be actually written like a Ki and NOT like a reversed Chi. However, because of the stroke sequence, the small circle joining the stroke 2 to stroke 3 is nothing but a simulation of the effect the brush would leave on the paper when it is moving from the bottom of stroke 2 to the top of stroke 3.
I hope this makes sense to you...