This design also serves a business purpose. You can see at a glance if someone has this week's pass without carefully checking the date. Our local bus company does something similar - the day passes have a random word for the day so you can't just print one at home, at least not without colluding with someone who's already bought a ticket.
Yup, that's the utilitarian reason for having a different design for each week. Most companies didn't go so far however. But even the monthly passes that I used in Romania during my youth were printed with different background colors every month (http://1.stpt.ro/forum/index.php?s=9978f358f69f69336b0cefcc5...).