Now that I look more closely into it, it appears to be keyed onto the cam shaft. If that keyed or pinned area cracked or broke, the pulley / sprocket may slip off alignment and make it impossible to set the timing according to the marks on the engine. Not that I have ever heard of this happening, but if the cam chain tensioner is good with the rub blocks are intact, and the chain is set to the right number of links between the sprockets and the belt is correctly mounted and routed through the other pulleys, idler and tensioners, that would be the only variable left to correct. Wouldn't it?.You do not remove the sprocket on a 1.8T for timing adjustments. Ever.