They are selected because of the Swedish expressions for those lines (translated): Spinning side and Sword side.
The correct English expressions are Distaff side and Spear side: [http://www.grammarphobia.com/blog/2008/04/the-flip-side-of-distaff.html The Flip Side Of Distaff]
If you don't like the images, I provide you with a set of replacement images:
| Individ's Paternal Line tree, Admin view (50%)
| Family's Maternal Line trees, Admin view (50%)