BRE Forward chaining problem when using Schema as vocabulary

I have a list of exams and grades of various students in it. I want to calculate topper for each exam using BRE. The structure of my xml message looks like this:

<Exams> 
    <Exam> 
        <ExamID>1</ExamID> 
        <Topper> 
            <StudentName></StudentName> 
            <StudentGrade>0</StudentGrade> 
        </Topper> 
        <Grades> 
            <Grade> 
                <StudentName>A</StudentName> 
                <StudentGrade>5.2</StudentGrade> 
            </Grade> 
            <Grade> 
                <StudentName>B</StudentName> 
                <StudentGrade>6.5</StudentGrade> 
            </Grade> 
        </Grades> 
    </Exam> 
    <Exam> 
        <ExamID>2</ExamID> 
        <Topper> 
            <StudentName></StudentName> 
            <StudentGrade>0</StudentGrade> 
        </Topper> 
        <Grades> 
            <Grade> 
                <StudentName>C</StudentName> 
                <StudentGrade>5</StudentGrade> 
            </Grade> 
            <Grade> 
                <StudentName>D</StudentName> 
                <StudentGrade>9</StudentGrade> 
            </Grade> 
        </Grades> 
    </Exam>
</Exams>

I am using following BRE policy to find the topper:

IF Exams:/Exams/Exam/Grades/Grade/StudentGrade is greater than Exams:/Exams/Exam/Topper/StudentGrade
THEN
        Exams:/Exams/Exam/Topper/StudentGrade = Exams:/Exams/Exam/Grades/Grade/StudentGrade
        Exams:/Exams/Exam/Topper/StudentName = Exams:/Exams/Exam/Grades/Grade/StudentName
        update Exams:/Exams/Exam/Topper

when i execute the above policy, The output which I get is wrong. Please see following xml:

<Exams> 
    <Exam> 
        <ExamID>1</ExamID> 
        <Topper> 
            <StudentName>D</StudentName> 
            <StudentGrade>9</StudentGrade> 
        </Topper> 
        <Grades> 
            <Grade> 
                <StudentName>A</StudentName> 
                <StudentGrade>5.2</StudentGrade> 
            </Grade> 
            <Grade> 
                <StudentName>B</StudentName> 
                <StudentGrade>6.5</StudentGrade> 
            </Grade> 
        </Grades> 
    </Exam> 
    <Exam> 
        <ExamID>2</ExamID> 
        <Topper> 
            <StudentName>D</StudentName> 
            <StudentGrade>9</StudentGrade> 
        </Topper> 
        <Grades> 
            <Grade> 
                <StudentName>C</StudentName> 
                <StudentGrade>5</StudentGrade> 
            </Grade> 
            <Grade> 
                <StudentName>D</StudentName> 
                <StudentGrade>9</StudentGrade> 
            </Grade> 
        </Grades> 
    </Exam>
</Exams>

Ideally, BRE should calculate Topper for each exam separately but some how It is setting the Topper by comparing all the grades in xml, irrespective of their alliances with their parent node.

If I see the test policy output, It becomes clear that for every assert of grade 2 conditions getting loaded in memory for the same grade i.e. one for each exam. This might result in output going wrong. Please see below:

FACT ACTIVITY 2/10/2009 9:57:57 AM
Rule Engine Instance Identifier: 47154365-d742-4322-996c-412bdecf9106
Ruleset Name: ExamPolicy
Operation: Assert
Object Type: TypedXmlDocument:Exams:/Exams/Exam/Grades/Grade
Object Instance Identifier: 49044892

CONDITION EVALUATION TEST (MATCH) 2/10/2009 9:57:57 AM
Rule Engine Instance Identifier: 47154365-d742-4322-996c-412bdecf9106
Ruleset Name: ExamPolicy
Test Expression: TypedXmlDocument:Exams:/Exams/Exam/Grades/Grade.StudentGrade > TypedXmlDocument:Exams:/Exams/Exam/Topper.StudentGrade
Left Operand Value: 5.2
Right Operand Value: 0
Test Result: True

AGENDA UPDATE 2/10/2009 9:57:57 AM
Rule Engine Instance Identifier: 47154365-d742-4322-996c-412bdecf9106
Ruleset Name: ExamPolicy
Operation: Add
Rule Name: FindTopper
Conflict Resolution Criteria: 0

CONDITION EVALUATION TEST (MATCH) 2/10/2009 9:57:57 AM
Rule Engine Instance Identifier: 47154365-d742-4322-996c-412bdecf9106
Ruleset Name: ExamPolicy
Test Expression: TypedXmlDocument:Exams:/Exams/Exam/Grades/Grade.StudentGrade > TypedXmlDocument:Exams:/Exams/Exam/Topper.StudentGrade
Left Operand Value: 5.2
Right Operand Value: 0
Test Result: True

can any one let me know how can I get the correct output in above case.

Thanks and Regards,
Navneet