I am guessing that the perfect 10 was reflective of his defensive abilities. To determine their rankings, CSB (now Central Scouting Service) ranks players on a scale of 1-10 in six different skill areas - skating, shooting, passing, defensive awareness and a couple others.
If Lehtinen was a perfect 10 in all areas, CSB would have had him No. 1 among Europeans, and he would have gone a lot higher than the fourth round in the weak 1992 draft.
Many prospects each year score a perfect 10 in at least one area. Joe Thornton, I believe, scored a perfect 10 in two or three skill areas (passing was one of them) when he went No. 1 overall in 1997.