Value changes when i select latest month

[vvvvvvizard]

This is my expression which shows the correct number for last month if no selections are made , or if any selection is made it still works , except  now when i select the latest month .

I noticed when i select the latest month the number changes slightly due to the KREF numbers not availble for the  latest month , but dont understand why its affecting the previous months number which is the expression below 

Sum( Aggr(Sum({<SPERIOD={$(y_Speriod-1)},FIN_YR_NO=, MTH_SHORT_DESC=>}DISTINCT no_rejection),Kref)

[Brett_Bleess]

Have a look at the following Design Blog post and see if that helps you resolve the why on this one, sorry I do not have something better for you:

https://community.qlik.com/t5/Qlik-Design-Blog/Set-Analysis-in-the-Aggr-function/ba-p/1463822

Regards,
Brett