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It was 40 (implying max efficient vote per day is 40) but now 10, which is 4x increased.

Ah didn't realized that. Curious too.

the hpp contains an initial value for vote_power_reserve_rate, but later when the code is running the value is changed to 10 only if we are working in the HardFork 19.

This is made in this way because the code must have a door to calculate all of the powers in the previous hardforks, because the first blocks were created in another manner.