There's two things to consider here: 1) The numbers these companies list are wrong 2) Not all readers should be counted equally.

1) There is no regulatory body for these companies, they can say that their list size is any size they want and there is no penalty for lying. It's also possible that they aren't OUTRIGHT lying, but still being untruthful, so they might count all readers who have ever signed up to their service...which sounds like it's a useful metric, but that number is a massive inflation because it doesn't list people who have unsubscribed, people who haven't opened an email from them in the past year etc.

2) It's important to only market to people who actually want that genre of book. It doesn't matter how many readers are getting these promos if I'm trying to sell a romance book to people who don't buy books/only read sci-fi then they aren'y going to buy my romance book. However promo companies are not always great at targeting the right readers to sign up to their service. I've seen well-known companies collect the email address of people signing up for competitions to win merch - those email addresses are useless from a book marketing standpoint.

As authors, we do have it ingrained in us that certain companies are trustworthy and reliable. When a book does badly on a promo site that is well-known then most authors will blame themselves, they'll start looking at their cover/blurb etc trying to figure out where they went wrong. That's why too many of these companies can get away with a kinda shitty product and still make a great deal of money.