Originally Posted by carleton

One factor is the use of narrow handlebars.

When your extended arms are far apart, your back goes down. When you bring them together (as with narrow bars), your back goes up. To make your back go back down as before, you need to extend the narrow arms forward using a longer stem (which would adversely affect handling) or with a longer top tube.

We are talking centimeters and millimeters here, but bikes come in 2 and 1 cm increments for a reason.