The Membership Dues numbers are pulling from different transactions within each membership. When running the reports by Gift Date, we're reviewing the information relative to the Gift Dates instead of the Membership Date. When gifts that are linked to memberships meet the criteria of the Membership Dues Report, the specific transaction linked to the gift is pulled in. The dues on that transaction record are calculated.

The Membership Statistics by Category Report runs based off of the current transaction's dues for any memberships that meet the report criteria. These dues are calculated regardless of whether or not the gift is linked to the current transaction.

For example: A membership with two transactions - a JOIN, and a DROP (the current transaction).
The JOIN transaction has a gift of $100 linked to it, and the dues are listed at $100 on the JOIN transaction
The DROP transaction has no gifts linked, and its dues are $0

Assuming the linked gift meets the criteria for both reports, we would see the following in each report: 

The Membership Dues Report shows: $100. This occurs because the dues come from the transaction linked to the gift itself
Conversely, the Membership Statistics by Category Report shows: $0. This occurs because the dues come from the current transaction on the membership

Because the membership dues on the current transactions don't have to be lower, they could also be higher. Based on the date range included in the report, this leaves time for membership prices to increase.