What is Target Server Memory (KB)?
To super simplify in conventional memory model SQL Server calculates something like target1 and target2 pages using below formula
Target1 = Current committed pages of SQL Server + ( Available Physical Memory - min (Total Physical Memory Pages / 20, Available Physical Memory Pages / 2))
ullAvailPageFile: The maximum amount of memory the current process can commit, in bytes. This value is equal to or smaller than the system-wide available commit value. To calculate the system-wide available commit value, call GetPerformanceInfo and subtract the value of CommitTotal from the value of CommitLimit.
If (Max Server Memory < ullAvailPageFile)
Target2= Max Server Memory
Target2=Total Physical Memory
Target Server Memory (KB) =Minimum (Target1,Target2)
So if AvailablePhysicalMemory is very high (or) when MaxServermemory is low then Target Server Memory (KB) would give you the MaxServerMemory else value derived from above formula.