Fewer homes were making their way through the closing process in September, according to the National Association of Realtors' latest update of the Pending Home Sales Index. While this was the second month in a row where this figure went down, reaching its second lowest point in the year, it remained more robust than the same time last year.

The Pending Home Sales Index fell 2.3 percent from 109.3 in August to 106.8 in September. The measure reached its lowest point in the year in January, when it stood at 103.7.

However, contract activity has also experienced year-over-year growth for 13 consecutive months. September's measure is 3 percent higher than the same month in 2014.

The Pending Home Sales Index is a measure of pending sales on existing homes, where a contract has been signed but not finalized. The sale is typically concluded within one or two months of the contract signing.

A measure of 100 indicates that contract activity is equal to that experienced in 2001, the first year the Pending Home Sales Index was calculated. The National Association of Realtors says 2001 had between 5 million and 5.5 million sales of existing homes, which is considered normal for the current population in the United States.

Lawrence Yun, chief economist at the National Association of Realtors, says a variety of factors may have contributed to the decline in contract signings.

"There continues to be a dearth of available listings in the lower end of the market for first-time buyers, and Realtors in many areas are reporting stronger competition than what's normal this time of year because of stubbornly low inventory conditions," said Yun. "Additionally, the rockiness in the financial markets at the end of the summer and signs of a slowing U.S. economy may be causing some prospective buyers to take a wait-and-see approach."

However, Yun also said he believes the interest in purchasing a home and favorable market conditions should keep pending sales at a healthy level in the coming months.

"With interest rates hovering around 4 percent, rents rising at a near eight-year high, and job growth holding strong—albeit at a more modest pace than earlier this year—the overall demand for buying should stay at a healthy level despite some weakness in the overall economy."

The Pending Home Sales Index dropped in each of the National Association of Realtors' four geographical regions. It was lowest in the Northeast, falling 4 percent to 89.6. However, this figure was up 3.9 percent from September of 2014.

The smallest decline occurred in the West, where the measure fell only 0.2 percent to 104.4 and remained 6.6 percent above last year. The index stood at 104.7 in the Midwest, 2.5 percent below August's number but a 4.3 percent year-over-year increase. In the South, pending sales dipped 2.6 percent to 118.3, 0.1 percent below last year's figure.

The National Association of Realtors says the Pending Home Sales Index is based on a national sample that includes about 20 percent of existing home sales. The organization says that the monthly level of contract activity provides a good measure of closed existing home sales in the following two months.