Why is total and target server memory never reaching the configured memory value?

I have sql server with 100GB ram, of which 90GB is allocated to sql server. 2 GB is currently free as shown in task manager. 90GB=92160MB (as seen in the sql server memory settings) I am monitoring the total server memory and target server memory perf monitor counters. I can see that the target server memory never crosses 90150MB, and the total server memory never crosses 89100MB. What could be the reason for this? My understanding was that target memory should always be equal to the setting configured in sql server memory settings. But why is this different in my situation?