Hi, the main reason for that is steem blockchain uses 3 day average for STEEM price. For example that's why when STEEM price increases, the post value do not jump instantly but slowly goes up. Even though yesterday/today STEEM was trading at $3.8 levels, currently the 3-day avarage is just at $3.3 - but it will surely go up very soon :)

Also there's small variance in payouts depending at which moment the payout was done - how much profit was generated up to the moment of payout and how much will be passed and added to the next payout.

The payout you received before you undelegated was generated by @tipU 2 days ago - when the average price was still fairly low. It already started to climb up and users will see it in the payouts :)