There's a dot missing at the edge of the uppermost "3" square. The solution assumes that this "3" block is a square, and only uses 3 of those 8 edges. If you treat the "3" block as a 3x2 rectangle, using 3 of those 10 edges, then there is a different solution.
Well, all missing dots are actually supposed to be missing, sorry for the confusion. Thinking back, I shouldn't have used cheap tricks like that to achieve uniqueness but I was new at authoring then.
IT is natural to be misled by that design,but the size and location of the '3' does indicate that it belongs to the 2x2 square only.Unfortuantely the location of the missing dot was such.