The way we calculate number of customers in the Executive Summary is different than how it's calculated in the Customers Deep Dives. In the Executive Summary, we count unique customers visiting your shop on a daily-basis. This means that if a customer orders twice in a day, they will only be counted as a single customer for that day -- all daily views should be matching with Shopify. When rowed up to weekly or monthly views in the Executive Summary, however, we are taking the sum of all daily unique customer values within that date aggregation. This means that on a weekly or monthly view, customers could be counted multiple times if they placed multiple orders within the week/month.
For example, customer John places 1 order on the 9/2. He then places a second order on 9/2. Our daily customer count for 9/2 would be 1 -- since we only take unique customers on a daily-basis. Now let's say John places another order on 9/4. Our daily counts would now be 9/2: 1 customer, 9/4: 1 customer. When rowed up to weekly (9/1-9/7), our customer count would actually be 2 -- since we take unique customers on a daily basis, we have 1 customer for each day, and then summed we get 2 customers within the date aggregation.
If you're wanting to get "de-duped" customer counts, looking in the Customers - Last Order Analysis dashboard will give you this.