All sorts of things can cause a variation the most common being a chip reorder of the same design. Colors may be off, different UV symbols or location, etc. What consists of a different chip versus a variation is harder to say. Most consider different UV a different chip for example. I actually collect all variations so I consider anything different between two chips as a different chip. Another example would be a manufacturering error such as missing insert or misprinted or off location inlay. Variations are normally lots of each chip whereas errors are typically one or a few chips only.