Caveat I'm no expert on Google ads. Never bought one, never plan to, never advertised anything at all on any service. But since I'm capable of doing a basic web search, I found:

https://support.google.com/google-ads/answer/6366577?hl=en

This is the process for determining which ads get run. The bid is only one of many factors, so as their support document indicates, the price you pay is often quite lower than the bid, which reflects a ceiling rather than a real sale price.

https://support.google.com/google-ads/answer/2580383?sjid=17...

This is their guidance on demographic targeting. Note there is no category allowing you intentionally target children. This doesn't mean advertisers can't figure out some way to do it anyway, but it means Proton can only sample from adults. Presumably, some probably very large number of the people who "use Google services" in your estimate are children, which childstats.gov indicates represent about 22% of all Americans. That makes it more like 195-235M adult users of Google services.

https://support.google.com/google-ads/answer/2464960

As indicated here, you don't pay to place an ad. You pay for clicks, so regardless of what you bid and who you target, Google isn't getting revenue for the number of placements you bid on, which is what Proton is sampling here. Presumably of the 250 x 0.78 to 300 x 0.78 million adult users advertisers are placing those $1605 average bids on, quite a bit fewer than 100% actually click on at least one ad.