Lots of nines means they have SLAs of no significant downtime. It does not guarantee that the SLAs can never be missed, just that there will be consequences when it is missed. Lawyers and accountants and sales folks will sort out the compensation payments that come back to the customers and try to negotiate relationships back to a healthy state.

That’s purely a choice by those claiming the nines. I’ve worked in companies selling software with more than 6 nines uptime (so that we can be more confident about the 5 nines). Not 4 nines uptime plus some money back for the other one.