The CrUX dataset only has about 18 million origins in it.
So the largest rank (rank = 50M) is incomplete and only has the leftover that didn’t fit in the rank <= 10 million bucket.
The amount of origins is slightly variable depending on how many make the eligibility requirements but think it’ll be a while before we see 50 million eligible ones! And if/when we do we’ll have a new rank = 100 million bucket with again left overs that didn’t fit in lower buckets.
So the largest tank bucket is not any indicator of how many origins are in CrUX versus how many are released publicly. It’s more just the largest bucket needed in the tank steps we use (10 raised to the power of x, with the half steps in between - so 1,000, 5,000, 10,000, 50,000… etc.).