This year, more Christmas gifts than usual may still be on their way. According to The Wall Street Journal, UPS, FedEx, and retailers including Amazon, Kohls, Groupon, and Walmart have all been unable to meet promises to deliver packages before the holiday, largely because of an overwhelming and unexpected increase in online shopping. "The volume of air packages in the UPS system did exceed capacity as demand was much greater than our forecast," UPS tells the Journal. While online commerce currently accounts for 6 percent of US retail sales, the Journal reports that the National Retail Federation believes the number shot up to an average of 14 percent of sales for this year's holidays.

Online shopping shot up more than expected

Making matters worse, the Journal reports that some retailers even pushed back this year's deadline for guaranteed delivery by Christmas, making them accountable for even more shipments. Though it's not clear exactly how much online shopping increased, it appears to be a good amount: IBM Digital Analytics reportedly measured a 37 percent increase in online shopping year over year for the weekend preceding Christmas, while Forrester Research believes there was a 15 percent boost in online shopping for the holidays — a slight increase over the National Retail Federation's estimates.

FedEx tells the Journal that its delays were not significant, though anecdotal reports suggested that deliveries had missed their deadlines. In a statement to the Journal, Amazon suggested that shippers were at fault for the late deliveries, though in an email to Amazon customers, UPS faulted Amazon for not readying and handing over the packages on time. Amazon is refunding shipping charges and offering a $20 gift card to customers with delayed orders, while Kohls — which says that only a small percentage of customers had packages delayed — will reportedly be paying for the entire cost of the delayed orders.

Walmart and UPS also tell the Journal that the delays have only impacted a small percentage of customers, though none detailed exactly how many. Groupon reportedly says that it delivered the "overwhelming majority" of orders, but is offering a $25 gift card to customers whose deliveries haven't arrived. Forrester Research tells the Journal that delaying packages guaranteed to arrive by Christmas isn't uncommon — usually occurring with around 15 percent of orders. Even if that number didn't rise this year, it may simply impact more people as the volume of online orders rises.

A compressed holiday shopping season may also have played a hand in this year's delays. This year, there were six fewer days between Thanksgiving and Christmas — the least amount of time in a decade, reports CNN. While that won't be a problem again next year, there's a good chance retailers will catch on to this holiday's mistakes around underestimating the demand from online shopping. Consumers picking up last-minute gifts, on the other hand, may just choose to shop in-store.