SQL Server 2008R2 memory issues

Hi Experts.

I have SQL Server 2008R2 installation,We allocated 102400MB(100GB) Memory   out of 128GB Installed.

But Task manager is showing  SQL Servr using 104 GB.Please some body let us know why SQL Server uisng more memory more than we allocated.

Please advise next step or recommendations if any 

OS :Windows 2008R2 64bit.
DB:SQL Server 2008R2 64bit.

Thanks in advance.

Regards,