First, I'd like to point out that you are writing the metric down wrong. Man/hour implies "men per hour (men/hour)", which is essentially a rate, not an output.
When you talk about the amount of total work performed, you're not referring to rates but instead are referring to output.
A man-hour or person-hour is the amount of work performed by an average worker in one hour.
Read more: http://www.answers.com/topic/man-hour#ixzz1L4B7a4N5
When you hear someone using the term "man hours", it's actually written as "man-hours" (not man/hours) and implies that it's the total number of "men" in this case multiplied by the total number of hours in the period.
So if I have 5 developers who work 8 hours per day, then 8*5=40 man-hours of work performed in one day. Whereas a rate involves division, output involves multiplication, so the two units will appear side by side, not one over the other with a division sign.
With that said, the Wikipedia article on Man Hours goes on to say that this type of measurement of output is really only effective for measuring simple tasks with simple dependencies that are also very predictable.
For instance, in the shipping industry, it's reasonable to assume that doubling your manpower will double your output, but in a chef's kitchen, adding more chefs won't make the water boil any faster.
Likewise, software engineering is not a simple production-based field with small, simple dependencies and simple tasks. Unfortunately, each engineer will bring different skills to the table, which involve different timelines and estimates. Moreover, the same developer who takes longer than other developers on one task may smoke those same developers on another task, as it's also about specialization, to some degree.