you are noticing the properties of the statistical normal distribution - think of it this way, as an example, for simplicity - on average, the market doesn't move on a daily basis, i.e. the average daily return on the dow is 0% - however, the standard deviation is 1%, i.e. there is a roughly 50/50 chance of it going up 1% or down 1%, on average - now the 1% off the approx. 10000 dow is 100 pts which is a typical daily range for the DOW, which can contract or expand depending on volatility - so, take the opening price as the middle value (i.e. if you do an analysis of the daily data, you will see that on average the market opens and closes in the middle of the daily range) - then the "average" range band will be between 50 pts above the open and 50pts below the open - it is not surprising to see a lot of commotion or "noise" around these levels as i presume they figure in prominently not only in statistical trading models but also in risk management models, i.e. hedgers, banks etc etc using 1 standard deviation level as thresholds for their risk control purposes. - you can also think of this 100pt range band not from the opening price, but from previous day's close, or from previous day's high or low - they may act as equally logical pivot levels... - all the best.
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