The National Retail Federation had some interesting numbers from Thanksgiving/Black Friday weekend.

Low prices helped keep Americans' budgets in check this weekend: on average, shoppers spent $407.02 from Thursday through Sunday, down from $423.55 last year.

That's one way to look at it. Here's another interpretation: consumers on average spent less than they did than the same period a year ago.

More than 141 million unique shoppers have already or will have shopped by the end of the big Thanksgiving weekend, up from 139 million.

More shoppers are always a good thing. But when you factor in how much money they spent, you get this: the number of shoppers over the weekend grew just 1.4 percent while average spending fell 4 percent.

Yikes. A 1.4 percent gain in shoppers hardly equals out a 4 percent decline in spending. Retailers are essentially brawling over a barely growing pool of Black Friday shoppers who are actually spending less on average than did a year ago.

In addition, there is one less shopping week this year because Thanksgiving fell on the last week of November. Suddenly, a season with an already low margin of (profit) error just got a little more perilous.