Because the different cams were designed for different firing orders. The 351 and the 302 had different firing orders until 1982. Since then, all the HO 5.0's had the same firing order as the 351's.
But who cares. It doesn't make any difference. You can put a 351 cam into any 302, you just have to set the firing order to match it. That's all.
Take care,
-Chris
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