Why is marginal cost not the cost of producing the last unit? I am slightly confused about this wikipedia article. en.wikipedia.org/wiki/Marginal_cost

It states that marginal cost is not the cost of producing the next or last unit.

Why is this the case? I thought that was precisely the definition of marginal cost.

I also hear marginal cost defined as cost of the last unit. What is meant by this? If quantity is increased by 5, is the marginal cost the cost of the 5th unit only?
The full quote from the cited reference (p. 181) is


  Marginal cost is not the cost of producing the "last" unit of output. The cost of producing the last unit of output is the same as the cost of producing the first or any other unit of output and is, in fact, the average cost of output. Marginal cost (in the finite sense) is the increase (or decrease) in cost resulting from the production of an extra increment of output, which is not the same thing as the "cost of the last unit."


I think the boldfaced part is the important part that the Wikipedia editor left out.

What you have in mind is correct. Suppose we can produce 4 hats for the cost of \$8, and 5 hats for the cost of \$15, then the authors are saying that if you choose to produce 5 hats instead of 4, then it would cost you \$7 more. That's the marginal cost.

They argue that it's not right to say that, having chosen to produce 5 hats, that the cost of producing the 5th hat is \$7.

To me, this is a very minor distinction.