If you have a ranking, say around 3-4 million, then you subscribe to Alexa’s advanced metrics plan at $9.99 a month, you will see your ranking go from millions to under 100k within 3 to 4 days.
Do you imagine your traffic has simply exploded overnight? Nope! It’s simply that their algorithm favors paid subscribers.
If you doubt it, feel free to pay for one month and see what happens.
They have the ability to offer accurate traffic tracking, but they refuse to do so, except it is paid for.