Hi Tibor,
The difference is that for the category=motors you have set Grid View on the category.
For the Category=legs you have not activated grid view
See the Plugins Docs Go to the section - Product Categories Grid View and follow the link to the WPEC wiki.
Re
And secondly: as you can see the thumbnails are not crunching to more lines, if I use more thumbnails in gallery they stay in one line pushing all content out of frame.. :)
You need to set the number of products to show per row - and that will depend on the size you set for the thumbnails - again read the docs and follow the link to the WPEC Wiki on Product Grid View settings
Once you have your WPEC Grid View settings correct for Product and Categories your problem will be solved.