There are two possible reasons for this. First, some hourglass sweater designs are designed specifically with no shaping on the front. This gives a slightly more relaxed look to the sweater, and allows me to use stitch patterning all over the front of the design without worrying about shaping in pattern. Coracle, Cushing Isle, Parker Head, and other designs are designed without shaping - on the design's information page, there will be wording to the effect of "this sweater is designed with shaping on the back of the sweater only".
Alternately, this may the best way to fit your particular measurements! For some people's measurements, CustomFit makes the front of your sweater wider than the back of your sweater at the bust. In some of those cases, this places more than half of your waist measurement on the front of your sweater, too. And once CustomFit gives you the appropriate amount of ease in the bust and waist, the front of the sweater becomes straight! The back shaping will easily make up for this; once seamed together, the sweater should still fit beautifully.
If you're nervous, take a look at this sweater on Ravelry -- the fronts are straight, with no shaping, and it looks great on her curves.