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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.