I'm sure you know what I am talking about. The reason for the difference in the renderings is a manuscript difference.
Modern translations follow what the original Hebrew of the passage says.
The variant probably originated as a copyist error when a scribe failed to take note that the subject of the verse had shifted from the woman to the seed of the woman. People notice this variant today because the expression found in the Douay-Rheims has been the basis of some popular Catholic art, showing a serene Mary standing over a crushed serpent. They prophetically foreshadow Mary and Jesus. Thus, just as the first half of the verse, speaking of the enmity between the serpent and the woman, has been applied to Mary, the second half, speaking of the head crushing and heel striking, has also been applied to Mary due to the manuscript variant, though it properly applies to Jesus, given the original Hebrew. This does not mean that the idea cannot be validly applied to Mary as well.