The Steem Blockchain doesn’t store how much Steem Power each account has. This is because the amount of Steem Power an account has, will go up over time, even if the account receives no transactions, or post rewards. This is on purpose.
When Steem was created, it was decided that the amount of Steem Power someone had would increase over time. This would encourage more people to power up their Steem, which is good for the platform. This is because it takes 13 weeks to power down Steem Power. This way, Steem is much more stable in value as more people are locked in to holding Steem.
However, it would be very inefficient for the blockchain to record every time someone gained any Steem Power, as the total amount is always increasing, albeit slowly. So, VESTS were created. This is what the blockchain stores, instead of Steem Power.
