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Its based on your recent game play. You must have wagered a lot more during the 2 weeks before you hit plat 3 than you did before plat 4.
True, recent game play affect the bonus. I ranked up to Plat 2 some time ago and I received $5 daily reload for 28days + the level up bonus $400. However no one knows about the exact calculation for the daily reload bonus after ranked up except Stake. All in all, it is a bonus we cant complain.
I do think that recent wagered is the most important part. I have seen people who have gambled a ton, like I mean millions, and then they stopped and got as low as 5 bucks as well, and I have seen people who wagered tens of thousands but brand new and got even more. This shows you that the most important aspect is how much you gambled since the last bonus, that is at least the most important part even if not the whole of it.
The "exact" calculation is definitely not known, but it is known that the more you wager the more you are going to get and it needs to be leading up to the bonus so that you would get even more in the end, otherwise it could be low amount.