For the same JFET why are there two curves in a Transfer characteristics graph?
Any given FET (at least when at a fixed temperature) will have just one curve, somewhere between those two limits.

Unfortunately, FETs are difficult to make consistently, and if you buy a number of 2Nxyz FETs, you can expect that their curves will not be identical, even if they are from the same batch. If they're from different batches, you should expect a wider spread. 

The manufacturer has gone to the trouble of characterising what he expects to be the minimum and maximum possible numbers likely with his manufacturing process, so that you have some idea over what range any FET of that type you buy will have. This is important, as your bias circuit must be able to properly bias any FET within that range. If your bias circuit does not handle that range of numbers, then you need to buy a better specified (probably selected, and so more expensive) FET, or to improve your bias circuit.