First of all, when you run the test for the second time VFP has cached the indexes and will re-use that information. To get an accurate evaluation you should close out of FoxPro and in some cases actually turn the machine off.
I've executed this instruction a lot of times, first one, then another... another, then one... :) And all results are the same.
Secondly, when VFP searches a table it reads the index(es) first and then retrieves the records it needs. When there is an index on a field with only a few discrete values (such as gender or yes/no) you are better off NOT having an index on that field. Then VFP will evaluate the other criteria, retrieve records, and make one final pass locally to evaluate the yes/no field.
The field that has a index is Caracter and size 10. It's a primary key in table A and I use this field to relations.
