Matt Levine at Bloomberg had an excellent explanation of the dynamics at play in his newsletter recently [1]. The bankers are incentivized to inflate the valuation in order to win the IPO mandate and all the fees that come with it. They then run into the awkward step of walking back that number to something that the market might accept.

[1]: https://www.bloomberg.com/opinion/articles/2019-09-09/we-mig...