Jan 20 (Reuters) - The market for existing U.S. homes softened unexpectedly in December, but 2021 still wrapped up with the strongest annual sales in 15 years, data from the National Association of Realtors out Thursday showed. Here are five standout facts from the 2021 housing market: HIGHEST ANNUAL SALES SINCE 2006 Sales for the year topped 6 million homes for the first time since 2006 and were up 8.3% from 2020"s total, the largest increase since 2013. Reuters Graphics Reuters Graphics FEWER HOMES FOR SALE The increase in sales came even as there were far fewer homes on the market. The year ended with just 920,000 homes listed for sale, the first time that figure has fallen below the 1 million mark since NAR began tracking it in 1982. The number of homes on the market each month has been lower than the same month a year earlier for 30 straight months. Reuters Graphics Reuters Graphics DOUBLE-DIGIT PRICE GAINS The short supply coupled with hot demand as the coronavirus pandemic reshapes preferences for where to live drove prices to record highs in 2021. On a year-over-year basis, sales prices have notched double-digit increases for 19 months in a row. Reuters Graphics Reuters Graphics PANDEMIC PRICING MIX SHIFT With prices rising fast, the mix of homes on the market by price range has shifted at breakneck speed during the pandemic. The share of sales in each of the top three price brackets -- $500,000 to $750,000; $750,000 to $1 million; and $1 million and up -- has roughly doubled since before the pandemic. The share of sales of homes in the $100,000 to $250,000 range, long the largest price bracket, declined at an unprecedented rate since the onset of the pandemic and now account for fewer than a quarter of all sales. Reuters Graphics Reuters Graphics TOUGH MARKET FOR FIRST-TIME BUYERS Surging prices make it all the harder for first-time buyers to compete for homes, especially with supply so limited. First-time buyers accounted for less than 30% of sales in the last half of 2021, the lowest in more than half a decade.