Why % difference not comparing with any month of previous year in a pivot

[Ram Mher]

I have a designed a pivot in which i want to comapre my all data with june 2014. result is reflecting only for the all motnhs of same year but when year change then resut is coming as NA. pivot sample is as below:-
Row LabelsSum of Ram
2014
May-15.10%
Jun
Jul-19.45%
Aug-17.74%
Sep-23.67%
Oct-8.92%
Nov-8.45%
Dec-2.25%
2015
Jan#N/A
Feb#N/A

Please refer the attached file and suggest the solution.

 

[Ram Mher]

my query is so simple but it seems challenge for the experts.

 

[Ram Mher]

dear hui sir

please provide any solution