Chad Callihan is overdrawn at the memory bank:
When you specify the max amount of memory that SQL Server can use, you may think that’s all the memory to be used for the buffer pool. That’s not the case. SQL Server will use memory not only for the buffer pool but also in tasks such as sorts, memory grants, and other internal tasks. These tasks can swoop in and “steal” memory from the buffer pool, causing more work for SQL Server.
Click through for some notes on the topic, as well as your reminder that “128 GB for the buffer pool” in Standard Edition doesn’t mean “128 GB total memory usage for SQL Server.”