NASA has been making it very clear that everything more that it does from now on is a bonus.
If you keep it framed like that, it's more exciting to have a successful mission & likely to continue to get funding, than if you set high expectations and fail.
This seems to be the more plausible reason. You keep the bar for success low so you keep getting funding for subsequent missions. Isn't that pretty much how every organization with public funding sets goals?
If you promise one run and fail to complete it, it's a disappointment, but eh, it was a high-risk mission anyway. If you promise a hundred runs and fail to complete even one, you'll catch a ton of flak. If you promise one and complete a hundred, you're a hero.
The math checks out that way, especially when one single mission is good enough to get the funding anyway. Everything else is literally a bonus, why promise ten when they're paying you for one?
What happens over a period of time in any organization if you consistently under-commit & over-deliver is that people stop believing in your "official" under-committed number ("It's just on-paper") and start creating their own informal "realistic" expectations (which are possibly different across stake-holders) and start taking (possibly mis-aligned) decisions based on these. As you may imagine, such situations tend to blow up from time to time.
Your boss wants to know how long you'll spend on a job; if you tell him 30 days and spend 30 days you're just good at estimation.
If you do it in 20, you're a star for doing it under budget!