One way to think of it: ZIPping or similar file-compression actions (RAR, LZH) make perfectly unique and perfectly reversible 'codes' that happen to be smaller (in most cases, not always) than the original. But JPEG makes an estimate of pixels. It's like cutting off the last three decimal places before writing down a measurement. When you build the object using these estimates, the result may well be good enough -- provided you have chosen the right amount of rounding. The real world, like the human eye, has a certain amount of built-in tolerance and adjustment. But when the estimate (JPEG) has a pixel valued at 3.14, there's no way to determine whether the original was 3.14159 or 3.14284. So a decompressed version may not exactly equal the original.

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