Most of our work is straight labour, no materials. So the profitability % is a simple calculation of labour cost vs invoiced amount.
I have found that if you end up invoicing extras that aren't included on the job it will skew the profitability % as it calculates based on the job price not the invoice amount. But it's still ok for a quick check. If the % dropped to say 70% then yes I'd look at the job time/labour cost and dig deeper to see what happened/what went wrong.
I would still back cost manually every so often as this will give you more historical data to see patterns etc. Jobber profitability % is useful for a quick "running check" but you need to take the reigns and dig deeper into your numbers to really be sure things are heading the right direction in the long term.