Buffer pool usage would not be more accurate as it is only part of SQL Server's memory usage.
There are numerous things consuming memory outside of the buffer pool. Some of this is explained in the documentation for memory server configurations - which also explains why it is not unusual to see SQL Server consuming more than the max memory settings:
Server Memory Server Configuration Options
max server memory controls the SQL Server memory allocation, including the buffer pool, compile memory, all caches, qe memory grants, lock manager memory, and clr memory (essentially any memory clerk found in sys.dm_os_memory_clerks). Memory for thread stacks, memory heaps, linked server providers other than SQL Server, and any memory allocated by a non SQL Server DLL are not controlled by max server memory.

Sue