ESV Bible: Translation of Mark 1:38

I'm reading Mark and I quite accidentally discovered a difference between my older ESV bible and the newer ESV Study Bible.
Mark 1:38 in my older ESV:

And he said to them, "Let us go on to the next towns, that I may preach there also, for that is what I came for." (bold mine)

The same passage in my newer ESV:

And he said to them, "Let us go on to the next towns, that I may preach there also, for that is why I came out." (again, bold mine)

I noticed it because I didn't recognise "I came out" bit that Jesus said. I'm guessing that the change was made because it's a more accurate translation, but I don't seem to have any way of finding it out myself. I don't know Greek, yet!

Compare against these other translations of the same passage if you are interested.