To simply put it.

There are 100% Het Pieds without Het Pied markings

There are 100% Normals with Het Pied markings

There are 100 % Het Pieds with Het Pied Markings

Judging a Het Pied by markings is like judging the sex of a snake based on the size of the spurs. Yea...It may accurate sometimes....but realistically it is not always going to work.