I think it is because the device is not really designed for projector use. The lower resolution output can cause problems with some projectors. But with others it will work wonderfully well. You can purchase the Apple video cable or borrow one and take your Ipad with you to a store that has demo units of the projectors and then see which one will work with your device. You can test applications by hooking the cable to the input on your digital tv to see what programs will transmit out with the cable. In fact you may wish to do this first with a borrowed cable before shelling out the money. After all a projector is pretty pricey if you find out that the programs you want to use do not allow video output.
There are ways to get the Ipad to output all video through the Apple cable, but it involves some tricky maneuvers and will void your warranty with Apple, so I will not discuss that here.