1 1/2 is between 1 and 2. We need to divide the space between each number into 2 parts, each of them with a length of 1/2.
2 1/6 is between 2 and 3. We need to divide the space between each number into 6 parts, each of them with a length of 1/6 (only the divisions from 2 to 3 are shown in the number line).