@ Eratas: That's due to certain actions between the two, and how she's learned things from other people.
Tabane has defined a mother (more like an older sister figure) as a woman who cares for your needs and emotions: her understanding links that to Laura after Chifuyu explains the concept to her in Layman's terms.
Rin, on the other hand, has to explain what's a lover, so she defines it as someone you personally care for with a passion, someone you want to spend the rest of your life with: And she links that to Kyuukai after Cecilia explains it to her, and again, in Layman's terms.
@ Zero: My top 3 list of favorite pairings for Ichika:
1. Charlotte
2. Madoka (after reading your fic
)
3. Houki