Thanks Rhino.
I've pondered this question for a while from time to time - haven't exactly lost sleep over it, though. What got me thinking was Faf (I think) padding Pup away the other day, and being declared not out on review because the ball pitched one centimetre the wrong way outside leg stump. Only about 48% of the ball pitched in line with the stumps, so it was an automatic "not out" even though he padded it away, and it would have hit middle.
Speaking as a left handed bowler, I think it's a bit rich that right hand batsmen get this luxury, as the natural drift of a left hand bowler is from leg to off, to a right handed batsman. The same ball kicked away by a rhb and deemed not out, could be out to a lhb if he kicked it away and it would have hit the stumps.
The leg theory suggestion is interesting, but I reckon balls that would hit the stumps (one pre-req of an lbw) would not constitute leg theory anyway.
Still puzzled, but more likely to dream about Toumpas/Viney/Hogan than this matter!