Anxious would-be home buyers have been watching mortgage interest rates finally begin ticking up again in 2017, after years of historical lows. And when the Federal Reserve raised the short-term interest rate in March, the conventional wisdom was that mortgage rates would follow suit-as they typically do.
But that's not happening this time around. Despite a Fed hike just last month and two more looming on the horizon, the average interest rate for a 30-year fixed-rate mortgage fell back below 4% for the first time since November, according to  Freddie Mac.
They were at just 3.97% as of Thursday, down from 4.08% the previous week and a high of 4.3% for the year, on March 16.
So what's going on?
"If you had an answer to that question, you'd probably make millions trading on Wall Street," says  Danielle Hale, managing director of housing research at the National Association of Realtors®. "Interest rates are really tricky to predict."
Yet even a small change in the interest rate can be a game changer for buyers. A fraction of a percentage point can add up to hundreds of extra dollars a year in mortgage payments. And that extra money can make a real difference in the kinds, sizes, and locations of homes that buyers can afford.

What's really driving down mortgage interest rates

Mortgage interest rates are  influenced by the Federal Reserve's short-term interest rates, but in fact they're more closely tied to the 10-year U.S. Treasury bond market. That's because as investments, bonds and mortgages are similar: Investors consider them significantly safer bets than the more volatile stock market.
So when investors got spooked by the rising stock market (fearing a downturn) and the unpredictability of the current U.S. administration, they put their money into bonds. And since mortgage rates are generally an inverse reflection of the strength of the bond market, when bonds are up, mortgage interest rates drop.
"Investors are a little skeptical because the stock market keeps climbing," says  Don Frommeyer, a mortgage officer at Marine Bank in Indianapolis. "They're looking for safe ways to invest their money, and they're going back to the bond market."
They're also not quite as optimistic about how quickly  President  Donald Trump's infrastructure plans will come to fruition, after having invested in sectors expected to profit from those plans. So they're turning back to bonds.
"Now people are re-evaluating and coming to the conclusion that [these promises] are going to take a lot longer than they expected," says Freddie Mac's chief economist,  Sean Becketti.

How long the downward trend might last

The lower rates aren't just confined to conservative 30-year fixed-rate loans. Rates averaged 3.23% on 15-year fixed-rate loans and 3.1% on five-year adjustable-rate mortgages as of Thursday, according to Freddie Mac.
"It's good news for people who are already in the market," says Hale. "Lower mortgage rates translate into a lower monthly payment."
But this situation might not last. With two more Fed hikes expected this year, mortgage rates are likely to fall back in line with their usual pattern. They're anticipated to keep going up gradually, Hale adds.
Of course, life can always throw us a curveball.
"There's so much uncertainty and volatility in the market," there's no way of knowing what the mortgage rates will do next, says Becketti.
Clare Trapasso is the senior news editor of realtor.com and an adjunct journalism professor. She previously wrote for a Financial Times publication and the New York Daily News. Contact her at [email protected].