Why doesn't the missing information errpr highlight the correct field on my form?

Missing information does not highlight the correct field.  On the screenshot below, I was missing the credit card number, but the form is pointing to the zip code field.  With some additional testing, I realized that the error box always points to the zip code field.  

The screenshot you uploaded is broken, but I was able to reproduce what you mentioned above using your form. The highlight at the bottom of the zip code is for the whole Payment field not the zip code textbox only.

You will realize that when you choose 'Credit Card' as payment method even if you enter the zip code, any other missing textbox will cause the error to display the same.

Let us know if you need further help. Hope this resolve your concern.