I have a new guess. Maybe it's the reward distributions? Rewards get created on a schedule, but they don't get distributed on a schedule. Reward distribution depends upon the activities of authors and voters.

During the last 20 days, the pending_rewarded_vesting_steem value declined from 524K to 506K, ~900 per day. None of this seems to be documented (outside of code) - and I'm not sure of the exact mechanism, but if I'm surmising right, there would have been a corresponding slowdown in virtual_supply growth(?).

So far, I can only compare in broad averages, but it seems that 900 per day average of rewards reduction and 950 per day of burned beneficiary rewards more-or-less closes the gap. Here's a graph with the pending rewards and the virtual supply (Note that the left & right scales are different).

Also, I've been running a check straight from the blockchain for about 22 hours. Here's what that yields (so far):

(Columns are: block number/virtual supply/pending rewards/virtual supply - pending rewards)

Starting values: 76462772 487895152.216 503395.264 487391756.952000
Last values: 76487132 487970204.933 508525.579 487461679.354000
Total change: 24360 75052.717000 5130.315000 69922.402000
New STEEM per block: (# blocks):24360 (Virtual Supply):3.080982 (rewards pending):0.210604 (Virtual Supply - Rewards Pending):2.870378
New STEEM per day: (# blocks):24360 (Virtual Supply):88732.281600  (rewards pending):6065.395200 (Virtual Supply - Rewards Pending):82666.886400

Guess I'll keep my eye on the pending rewards for a couple weeks and see how things play out.

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Update for my own use:

Saving this somewhat related link from 2017 for myself later. Here's a little shell script to calculate the rewards_per_rshare and output it in CSV format:

while :
do
   INFO=$(curl -s --data '{"jsonrpc":"2.0", "method":"condenser_api.get_reward_fund", "params":["post"], "id":1}' https://api.steemitdev.com \
             | jq -Src '.result | .recent_claims,.reward_balance,.last_update' | sed -n '$!N;$!N;s/\n/ /g;P;D')
   recentClaims=$(echo ${INFO} | awk '{print $1}')
   rewardBalance=$(echo ${INFO} | awk '{print $2}')
   rewardPerRshare=$(echo ${rewardBalance} ${recentClaims} | awk '{printf "%0.18f", $1 / $2}')
   echo ${INFO} ${rewardPerRshare} | sed 's/ /,/g' | sed 's/,STEEM//g'
   sleep 3
done

@steemcurator01, is there any chance that this API can be reactivated at SteemDB? It stopped updating in April of 2021.