Help: Original I-797 for H1B visa interview?

The list from embassy's website shows that you have to bring the orignal I797 form to show your H1B is approved when you go to have the visa interview. However, I don't have the orignal whole form. I have the lower part the I797 form, and the upper part is kept by my employer. I have the copy of the whole form. Actually, it says "the pentitioner should keep the upper portion of this notice" on this form. I don't understand why they still ask for the orignal form. Does anybody have any idea? I know it is stupid.