It may have been a result of the application of the IHS to the chip. Forced in place and glued, the thin center would have bulged slightly, but it may have been necessary to insure a good tim joint under the IHS, then the pad negated the bulge problem without the expense of another machining step to flatten it again. The nickel coating would have to have been done after it was flattened out on the chip also. It was probably a cost vs performance juggle. They kept the cost down and still got reliable performance.
All fun speculation but the spreader is bulged, that's a fact. We know that a thinner tim joint is better also. Kept as stock you are forced to either accept low surface area contact with a thin layer of goop, or use a heavy layer. Something to look for right away when AMD starts putting them on their chips too
