Steps vs miles

The other day both my daughter and I walked 18,000 steps, but her watch said she walked 8.5 miles, mine said I walked 6.75 miles. She is 2 inches taller than me, but other than that why is there such a big difference in the miles walked? This is important because our challenge goal for the month is how many miles we walked, not how many steps we walked and I walk in place a lot (while reading a book).