Power Pivot Distinct Count

[dap]

Hello,
I have a data set that include a list of customers with the date that they visit. I'm trying to create a pivot table that shows the count of distinct customers per month for 3 years. Now I did this with Power Pivot but, when I take a list of customers for a month and delete the duplicate values, I get a different number. It's less. This seemed to be the case for every other month I check this for. Why would the numbers be different? How can I reconcile this? Thank you for your help!

 

[GlennUK]

The numbers would not be different, if all the information you have given us is accurate. Show an example before the change, and the same after (data and Pivot result in both cases).