May I ask what type of system screen shot was taken from? If you are on a Mac running OS X 10.10.x (Yosemite upgrade), your number format options would be as follows ... Note that the Accounting format does not place parentheses around the negative number. See "Sample" on the screen.
I tried to replicate your example, but under "Custom ... ", parentheses around negative numbers were not offered as an option, as pictured in your response. To check whether this anomaly just affected my THREE Macs, I traveled to the Apple store and tried to format a negative number with the Accounting format. Sadly, the negative number was formatted with a minus sign out front, instead of surrounded by parentheses as it should have been.
I recently spent some time with a Microsoft Office for Mac team engineer, and at first he could not replicate the anomaly. When I inquired as to what OS he was using, he responded he was on a windows PC. Windows PC? Troubleshooting a Mac problem? I noted he was a member of the Office for Mac team, and shouldn't he be working on a Mac? So he found a Mac, and sure enough he suffered the same problem.
Microsoft issued an update to Microsoft Office for Mac 2011, which I installed, shortly after the Yosemite OS was released, to insure the Office for Mac programs operated as expected under Yosemite. Unfortunately, they Accounting format was lost in the transition. It has been a known problem since October 2014. Microsoft has yet to remedy the problem.
I can only conclude Microsoft is not interested in correcting this, which is unfortunate since most Excel users use the program in the financial area where the Accounting format is necessary. I cannot believe it is a difficult correction to implement. It shows a callous disregard for its users by Microsoft. I will not be upgrading the Microsoft product beyond what I have now.