BAYES_* missing in some mails why?

[Jesper K. Pedersen]

I found that some mails do not have any BAYES_* (e.g. BAYES_10, BAYES_80 etc) 
scores in the X-Spam-Status header field.

Is this a feature or a bug? And if it is a feature, can it be configured, so I 
always get it?

Thanks
Jesper