This is entirely normal and makes sense when you look at how the voice works.
The vocal folds, commonly referred to as vocal cords vibrate to create the sound. Similar to a guitar string, the more tension on them the higher the pitch. The muscles primarily involved in this are the cricothyroid (CT) and thyroarytenoid (TA) muscles. As these muscles contract the pitch increases.
To obtain the lowest note you can sing you relax both of these muscles completely. Since they are relaxed completely the anatomy of your throat determines the lowest pitch.
At the other end, to produce your highest note you contract your CT and TA muscles as much as you can. The conditioning of the muscles and the flexibility of your vocal folds is what determines your highest pitch. These are things that can vary a lot more than the lower end so the high end is going to be a more gradual change.
Note: I found one paper (.pdf) from 1972 that makes the assertion that contraction of the sternohyoid muscles causes the pitch to lower, but I can't find any other source to support this hypothesis.