Yours appears to have a color misregistration that causes the look of the lettering (and the picture) to appear slightly blurred.
In fact, if you flyspeck that stamp a bit closer, I think you'll find another anomaly ... that the halo over the Christ Child is broken in the center!
Nonetheless, back to your question about that mysterious horizontal line. It prompted me to check my stockbook and I came up with this one, which I thought at first was damaged, but after looking at your copy I'm not so sure ... as I never noticed it before:
