Another quarter, another earnings beat. Under Armour's (NYSE:UA) off to the races, but judging from how they reacted to yesterday's news, investors are looking a little ... slow.

On Thursday, the sportswear maker and Motley Fool Rule Breakers recommendation beat estimates with a quarter that featured 24% revenue growth and 76% improvement in per-share earnings. Inventories went down, cash flow went up -- and the stock went nowhere. (Note to Under Armour shareholders: American Express (NYSE:AXP), Apple (NASDAQ:AAPL), and Google (NASDAQ:GOOG) shareholders also got no love after solid reports, and send their sympathies as they feel your pain.)

Why not? According to the pundits, it's the same reason this time as last quarter. Although fiscal 2010 revenue could reach $960 million (5% better than analyst forecasts), management predicted profit will grow only "in line" with that sales increase -- beating the Street by "only" 3%. Investors wanted to see Under Armour make bolder promises, it seems, and they're punishing the stock for management's conservative stance.

The more things stay the same, the more … they underestimate Under Armour
Interesting. And it reminds me -- three months ago, in discussing a similar post-earnings sell-off at Under Armour, I surveyed the financial press's criticism of Under Armour, and concluded: "Under Armour beat estimates in Q3 [and raised] guidance for the year. But guidance didn't get raised enough to account for both better performance in Q3, and further great news in Q4." This explained the sell-off.

Back then, I challenged investors to ponder the possibility that Under Armour was pulling a rope-a-dope, lowballing its projections so as to make an outsized earnings beat more likely -- just as it had done in nine of the previous 10 quarters. Just as it did yesterday. (So make that 10 out of the previous 11, now.)

Foolish takeaway
Seeing as the stock currently sells for less than it fetched four months ago, you might think investors would begin to wise up, and get hip to the bargain they're being offered. So far, however, it doesn't seem to be working out that way.

To ensure investors today don't fall prey to the same error that tripped up their counterparts in October, let me make this crystal clear: Under Armour is a winner. It doesn't overpromise; it makes promises it can keep, and then exceeds them when it's able.

Oh, and it leaves Nike (NYSE:NKE), Timberland (NYSE:TBL), and Columbia (NASDAQ:COLM) eating its sales dust.