Market fear is rising, but it would appear that someone forgot to tell the market.

The CBOE Volatility Index looks set to log its seventh straight rise Tuesday, in what would be only the third time the VIX has managed that long a string of gains in the past 20 years. But what is even more unusual is how the is responding to the surge in expected volatility.

The S&P and the VIX tend to have a strong inverse correlation, which makes sense, since the VIX roughly tracks nervousness over potential market downside. But in the current VIX rise, the market has stayed remarkably stable.

In the six days ended Monday, which saw the VIX jump 56 percent, the S&P 500 was down less than 1 percent. That is in sharp contrast to the average S&P drop of 6.7 percent during six-day periods in which the VIX has risen 55 percent or more. In fact, never before in the past 20 years of market data has the S&P been down this little in a six-day period that saw the VIX rise by this much.

The VIX is not the only potential measure of fear that has seen a dramatic rise of late. Gold prices have increased about $40 per troy ounce in the same period. And bitcoin's considerable rise over the past few days has once again put the so-called cryptocurrency in the headlines.

So why is the fear dog barking while no one is at the metaphorical market door?

"People are buying puts and calls because they don't know what the [heck] is going to happen," Dennis Davitt, partner at Harvest Volatility Management, said Tuesday. "There's just great uncertainty."

That is, risk assets around the world are fast approaching a purely binary event — the British referendum to exit the EU. While a win for the "leave" movement once appeared a considerable long shot, polls have shown substantial growth in those who say they will vote for separation, and on Tuesday afternoon online betting market PredictIt implied a 43 percent chance that leave will win.

For this reason, it naturally makes sense that both bullish and bearish options have become more valuable, as a decision to stay removes a major potential risk to the global economy, and a decision to leave could have seriously negative implications.

But there is another reason the mild market reaction makes sense. The rise in the VIX has come off of markedly low levels, and even at its Tuesday highs, the VIX was not much above its classic long-term average of 20.

This means that the VIX may be catching up to market sentiment more than portraying a major shift in risk appetites.