Quote from FullyArticulate:
Keep in mind that futures trade for 15 minutes after the close of the cash market. So, if someone had great earnings after the cash close yesterday, the S&P futures might close up 2 extra points. The next day, the futures market will appear to be up 2 points fewer than cash simply due to when the sample was taken.
A better measure, as whitster said, is the PREM, $PREM, PREM.X, or whatever the heck your data source calls it. It measures the premium of the futures contract over the cash index and will fluctuate quite a bit during the day.
Another point to remember is the premium will vary based upon the time remaining in the futures contract. The premium today, and the premium on March 1st will be substantially different.