Windows Server task manager displays much higher memory use than sum of all processes' working set size
Adding up the memory usage of all processes will not generally produce meaningful results. That will leave two major users of memory unaccounted for, the system cache and the standby list. You cannot account for memory usage by simply adding up a list of numbers. The memory management system is far too complex for that.
Adding up working sets also multi-counts pages that are shared across processes, like code from system DLLs and executables started more than once.