As we all know, the bridge pickup on the strat is angled and so the pole piece magnets do not sit directly under the high E and low E strings.Whilst making minor adjustments to my guitar, I noticed that as a result of the strings being over the edges of the magnets that the up and down strokes produced a very noticeable difference in tone on the Low E and High E strings.Particularly the low E.The initial direction the pick pulls the string was making it sound full in one direction and thin in the other.Similar with the high E string.As both of these strings sit right near the magnet edges.Funny how I never really noticed before just how much difference there is!
Try it.It may be less of a problem on guitars with the closer string spacing bridge but is very noticeable on my 63 strat.