Eh, these are people that had a look at Stripe's financials and decided to invest in it at that valuation. I'm not going to speculate on their actual numbers, but I trust that Sequoia and co have a good idea what they're doing.

Also, Stripe's "$1.75 billion" valuation is misleading because the round was for preferred shares, which have privileges (and lower risk than common stock). We saw something similar for AirBnb, which Marc Andreessen pointed out in a series of tweets:

https://twitter.com/pmarca/status/457017580385873921

Square's valuation was based on common stock.