Imiona w Polsce / Names in Poland

One of the most interesting things that I discovered first time I was in Poland was how Polish make shortcuts for names. The rule seems to be: the shortcut has nothing in common with the name itself, it's totally different ;)) Of course, not for all of them.
There could be incredibly a lot of versions for one name. For example, for the name Maria, there can be: Marysia, Maryśka, Marysieńka...
For Piotr, there can be Piotrek and Piotruś :) Sometimes, you can tell the age of a person by the way he's called. There was a funny situation like that with my Piotr. He has two older brothers, so they often refer to him as Piotruś (I also do that:)), although he's quite big :))) This way, the girlfriend of one of his brothers expected him to be a little guy, and when she met him, she was surprised ;))) She realised that "little" Piotruś had 90 kilos and 183cm :D
But some of the versions are quite odd. For example, for Aleksander/Aleksandra, there are Olek/Ola and Oleńka, and for Joanna there is Asia :) Also, for Andzrej, there are these versions: Jędrek and Jędruś. And so on...
Unfortunately, or fortunately, we didn't find an explanation, but it's good to know :)