Question - Two Different First Days?

I am trying to sort through a ton of first day stuff (letters and post cards) and came across something weird.

I have the top one as a "hinged pair" (with one unused, supposedly for reply) of Fist Day Issue of the 4-cent "New Postal Rate" dated 7/31/58.

I also have the bottom one as a single, used First Day Issue of the 4-cent "New Postal Rate" dated 8/1/58.

Why two different dates for the same FDI embossed post card? I'm supposing that this was not unusual although I have numerous other First Day of Issue pairs and they all match up.

I admit my lack of knowledge as I am trying to get through a large "attic find" that was inherited.

Thanks in advance!

The top card is UY17 and the bottom card is UX46. The top card is a paid reply card that was sold to the public as two unsevered cards.

Thanks! It's obvious that I have a lot to learn. These looked identical to me except one was an unsevered pair.