Hey! Interesting question, I was wondering about this for a long time, too. With the 19 meals a week, this is because on the weekends students tend to sleep in and thus brunch is offered instead of just lunch. With the 14 meals a week plan, that $135 is so you can spend it at any of the on-campus restaurants/eateries (of which there are many). A big lunch at a place like Treehouse is about $5.
There are often lots of events which also supply free food - society recruitments etc. To add to this, most students will buy snacks for their dorm rooms, and many will go into Palo Alto/Stanford Shopping Centre to get food occasionally, too.
If any other Stanford students have had a different perspective to add to this, please do so! Many of the people I’ve talked to finish their 14 meal plan with a few swipes left at the end of each week, too.