Different Performance Chart pattern between Overview and Advanced

Hi all,

I try to get the performance chart for memory utilization in one of VM. I collect it for past one year report (16 June 2014- 16 June 2015). I try to collect from VM's Performance tab- Overview and Advanced. But, I found that two chart show different pattern especially after 2 May 2015. (pls refer to attachment). Which chart will be more accuracy? Any other best way to collect VM utilization report?

Thank you

Regards

Ho