For vendors, trying to predict how much tea you will sell in a year is pretty much a loosely educated guess. Most large vendors are also wholesalers. At that point your accuracy depends on the purchasing (sometimes significant purchasing) of other retailers. Land a few big new accounts in a year and all your guesses go out the window.
Long story short, the customer gets upset if you run out of Sencha two months early, and gets upset if you still sell last year's Sencha two months longer than the competition. There is no easy answer, but it's certainly a fair question to ask and I completely agree with the "buyer beware" statement. Unscrupulous retailers and wholesalers will undoubtedly unload old stock on unsuspecting retailers and consumers.
Two final thoughts: First, beware the freshest teas at the start of the season. If the are shipped by air they may be 50% to 100% more expensive than they should be (or would be shipped by sea and arriving a month later). If they are selling for the same price, you may be getting a much lower quality tea!! No easy answers here either. Again, buyer beware.
Second, there is a big difference between "harvested this year" and received, packaged or blended this year. The gardens at source ALSO sell last year's tea at times. Come April, I can guarantee you that some retailers will be selling 2009 teas that were air-freighted to arrive in the US at the "height of freshness"! The retailer will be proud of themselves for getting a great deal. You'll be paying double for last year's tea.