Job margin is the profit left on a single job after the labour and materials that job consumed. Average job margin is that figure across all jobs in a period.
It answers a question a monthly profit and loss cannot: not whether the business made money, but which work made it.
Why the average hides things
A healthy average can sit on top of a wide spread. Some jobs carry the business and others lose money on every visit, and averaging them together makes both invisible. The spread is usually more useful than the average — if one job type is consistently below the rest, that is a pricing or scoping problem with a specific name.
What it needs to be accurate
Labour and materials have to be attributed to the job rather than swept into overhead. A business that books all wages as a monthly expense can see profit margin but cannot see job margin, because nothing connects the cost to the work.