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New Month Rebalance?

I was wondering why they rebalance at the beginning of a new month? In other words, the close on the last month for ESH8 might be 1335.25, but tonight could open at 1340.25 and it might say +1 instead of +5.

Do you know why they do it and how they arrive at the new number?
The reason why I ask is because on my Alaronline Trading Platform, on Friday it showed last price 1331.25;with bid 1335 and ask 1335.75.
But today it shows last 1369.00 with open 1358.75, High 1370.50, and Low 1357.00.(and Previous 1331.25) It shows bid 1368.75 ask 1369.00.
There might be a very simple explanation for this. The close could be either (1) the settlement price or (2) the last traded price. These are usually different. So you might be looking at one for the close but the price the following day is showing the change off the other. (I'm just taking a guess at what it might be - I don't know for sure.)