In general, you can only expect the interval between the rising and setting of a celestial object to be similar to that for the sun on that day if the object is near the ecliptic. Perhaps the most obvious counter-example is the pole star, which, being closely aligned with Earth's axis, never sets on Cupertino.

According to the same site, however, the comet is currently in Leo, which is close to the ecliptic and is currently setting before the sun, so I cannot see this being the explanation here.