So, just getting back to the topic - why does 100m make a difference why not 200m - how far "just over the border" are you allowed to be for it is not acceptable. I don't get it.

Unfortunately rules are rules and it is each club's responsibilty to check players' addresses before they are registered by referring to detailed boundary maps supplied by the SANFL. It is not that hard.
Mind you I can tell some horrific stories about some of the hills & country boundaries where they are very difficult to follow and clubs have mistakenly not contatced good players because they thought they were out of zone and realised a year or two later and were lucky that the club who they thought they belonged to had not already contacted them (one example the guy eventusally got drafted - in 2008 - and the other played SANFL state last year).
And then there is the story where a club knew that the boundary went through the family's property and technically the player was out of zone but they never told anybody and signed him up. Because they never did their homework, the real club never knew about it to this day - he went on to play league football.