There's always backroom negotiations going on with investments like these. Private valuations are normally hyped-up, and with the current batch of AI companies, 100x so.

I assume Anthropic said something like "We'll give you 3% of our company for $30B, since we're valued at $1T now! So cheap!", and Google immediately came back with "Hell no. We'll give you even more, $40B... but it's for 11% of the company. Take it or leave it." With all the issues they're having, what leverage does Anthropic have at that point?

Basically, Google made them an offer they couldn't refuse.