different results on frontend and backend when filtering on empty field

I’m very confused about a selection I want to make on the backend. Somehow it does not work out like it should. The situation is as follows:
• There are 632 members that are approved
• There is a field where is marked when they have made a donation: this is a text field, although the input is a date that is always like 01-01-2017.

On the front end I have a shortcode that filters for members that have not yet made a donation;
‘donation_2017~&type=member&approved=yes’
The result for his is 101. So there are 531 members that have made a donation.

When I filter on the backend, I use the same selection, so
1. “type” is equal to member
2. “approved” is equal to yes
3. “donation_2017” DOES NOT contain “201” (so I won’t miss the donations that were made in december 2016)

I would expect all 101 record with an empty field “donation_2017” to show up, but I only get 61 record. When I filter (in step 3) for “donation_2017” DOES contain “201”, I get the 531 record all right. So somehow there are 40 record that do not show up on the backend, but they do show up on the front end.

I downloaded a csv to see which records are missing, and tried to find out what could be wrong with them, but I cannot see anything strange. When I filter (still on the backend) on only the first 2 steps, and sort the list on the field donation_2017, and count the records with this field empty, there are 101 record all right.

I also tried to filter on “donation_2017” is equal to (nothing, that is: leaving the search field empty), but that does not give the expected result.

I don’t see any logic in this. Of course I could stick to filtering on the front end, but as there are also others that use the backend I would really like the system to work for “dummies” (excuse my language). Could you please help out?