Why is a contribution given a financial type different from that allocated to the contribution page used?
I am answering the question posed in the title, not dealing with the problem of debugging the Paypal problem.

When you define a membership type you specify a financial type.  That become the defualt financial type for that membership type. Unless you use price sets, the "default financial type for that membership type" will be recorded against the membership contribution no matter what you have chosen as the financial type for your contribution page. This is explained at the bottom of the help message that appears if you click on the ? associated with the Financial Type when you are setting up your contribution page.


  NOTE: If you are using this contribution page for membership signup and renewals, the financial type configured in the membership type settings will be used when recording the membership payment. The financial type selected here will only be used if you are also collecting additional contributions (as configured on the Membership Settings page).


If you need renewals to have a different financial type from new memberships (as we do) then you will need to use a price set for you renewal page specifying the renewal financial type at the line item level.  You will also need to extract financial data at the line item level, not just at the contribution level.