Drools invokes superclass static method

package test;

public class ARef {
    private String ref;

    protected ARef(String ref) {
        this.ref = ref;
    }

    public static ARef valueOf(String ref) {
        System.out.println("ARef> " + ref);
        return new ARef(ref);
    }

    @Override
    public String toString() {
        return ref;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((ref == null) ? 0 : ref.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null) return false;
        if (getClass() != obj.getClass()) return false;
        ARef other = (ARef) obj;
        if (ref == null) {
            if (other.ref != null) return false;
        } else if (!ref.equals(other.ref)) return false;
        return true;
    }
}

package test;

public class BRef extends ARef {
    private BRef(String ref) {
        super(ref);
    }

    public static BRef valueOf(String ref) {
        System.out.println("BRef> " + ref);
        return new BRef(ref);
    }
}

package test;

public class Criteria {
    private BRef ref;

    private boolean ok = false;

    public Criteria(BRef ref) {
        this.ref = ref;
    }

    public BRef getRef() {
        return ref;
    }

    public boolean isOk() {
        return ok;
    }

    public void setOk() {
        this.ok = true;
    }
}

package test;

import java.io.StringReader;

import org.drools.RuleBase;
import org.drools.RuleBaseFactory;
import org.drools.StatelessSession;
import org.drools.compiler.PackageBuilder;
import org.drools.rule.Package;

public class Test {
    private static final String DRL = "package test\r\n" + "dialect \"mvel\"\r\n" + "import test.*;\r\n" + "rule \"myRule\"\r\n" + "when\r\n"
            + "  criteria:Criteria(ref == BRef.valueOf(\"toto\"))\r\n" + "then\r\n" + "  criteria.setOk();\r\n" + "end";

    public static void main(String[] args) {
        System.out.println(DRL);
        try {
            PackageBuilder builder = new PackageBuilder();
            builder.addPackageFromDrl(new StringReader(DRL));
            Package pkg = builder.getPackage();
            RuleBase ruleBase = RuleBaseFactory.newRuleBase();
            ruleBase.addPackage(pkg);

            for (int i = 0; i < 1000; i++) {
                Criteria criteria = new Criteria(BRef.valueOf("toto"));

                StatelessSession session = ruleBase.newStatelessSession();
                session.execute(criteria);

                System.out.println("[" + i + "] " + criteria.isOk());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

package test
dialect "mvel"
import test.*;
rule "myRule"
when
  criteria:Criteria(ref == BRef.valueOf("toto"))
then
  criteria.setOk();
end
BRef> toto
BRef> toto
BRef> toto
[0] true
BRef> toto
BRef> toto
[1] true
BRef> toto
BRef> toto
[2] true
...
BRef> toto
ARef> toto
[109] false
BRef> toto
ARef> toto
[110] false
...
BRef> toto
ARef> toto
[998] false
BRef> toto
ARef> toto
[999] false