Each airline has its rules. Given that you specifically compare against the depth than they must have specified a size for each dimension and hence you have to be without that limit. When the linear size is considered, airlines write their rule in terms of linear size.
It is most common to specify a limit per dimension and each dimension must be up to the stated limit. Very few airlines I know of specify in terms of linear size but some do, mostly for personal items, rather than larger ones. So, I suspect your case does not fall within the rules but the only way to verify is to have the airline rules shown.