An explanation for this strangeness? Epi didn't work?

So I'm reading more and more about labour to get ready for #2 and it's reminded me of something strange that I never figured out. I had an epidural with DD after a looong and painful early labour with the flu (I was something like 4 - 6 cm when I had the epidural). Initially, they gave me the epidural and then put something else, some kind of narcotic, in the epidural space to 'top it up'. I didn't feel any contractions of my uterus as such after that but gradually I did feel DD decend (this was absolutely agony) and also felt her crown (this wasn't painful at all).

Isn't this a bit strange, that the medication would block out the pain of contractions but not of the decending and crowning?