NEW YORK (CNNMoney.com) -- Reading the government's weekly report on oil inventories used to be pretty simple. If inventories rose, prices fell. If stockpiles fell, prices rose.
Econ 101. Supply and demand.
That's no longer the case.
Oil prices still sometimes make big moves right after the inventory figures are released each Wednesday morning.
But then after an hour or two prices sometimes give up their gains, or losses, and in some cases turn completely around.
"It's called a fade day," said Peter Beutel, a veteran trader who's now an oil analyst at energy consulting firm Cameron Hanover, referring to the price reversals. "It happens on about 70 percent of report days."
And then to make matters more confusing, Thursday's price swings are more and more being attributed to the inventory numbers from the previous day - a puzzling explanation given traders' tendency to send prices soaring or plummeting on the first bit of news.
The fact that oil prices don't always behave rationally following the inventory report is clear. The reasons they don't are less so.
One analyst said reading the report in a bull market - when oil prices have been climbing for months or years - is fairly easy, as the numbers will be strong enough to continue the upward momentum, or weak enough cause a selloff.
But in the current market, where oil prices have gone from over $78 to below $50, then back to $65 all in the last nine months, figuring out the market's current trend is more difficult.
"You get that churning, that inconsistency, because it's in the context of a larger sideways market," said Tim Evans, a futures analyst at Citigroup. "It's almost like it's planned chaos."
Plus, Evans said, the inventory reports themselves have become more mixed, perhaps showing rising stocks of crude oil but falling gasoline inventories.
Another analyst said much of the buying or selling centered around the inventory report actually occurs before the numbers come out.
"We do operate a bit on a 'buy the rumor, sell the news'" mentality, said John Kilduff, an energy analyst at Man Financial in New York. "Sometimes it's not as bullish as some people had hoped."
Cameron Hanover's Beutel said a lot of the price movement surrounding the inventory report is done by technical traders, who basically use the report as an excuse to buy or sell.
"Fundamental traders react to it, and then the locals (traders on the NYMEX floor who buy or sell based on certain price points) take advantage of it," he said.
As far as the inventory numbers moving oil prices a day later, as is often reported in the media, Beutel's not buying it.
"There are no delayed reactions," he said, saying price movements the following day attributed to the report are more likely due to technical trading. "If the market goes higher for technical reasons, the editors don't want to print that. What they are doing is telling little white lies so people think they understand what's going on."
But with three experts giving three different reasons why prices don't always react to the inventory report as expected, it's not certain anyone really knows what's going on.
Happy trading. 
