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The total SmartRewards pool is defined to be 15% of the BlockRewards which is defined to be "5000 * 143500 / BlockHeight" which decreases with each block so the total SmartRewards pool decreases over time which means that less SMART is given away each month as SmartRewards. Additionally, the total amount of SMART increases over time (inflation) so over time there is an increasing amount of SMART around that can compete for the decreasing SmartRewards pool.