There is a couple of reasons.
1. the makeup of the gas you are using, some makers use excess of sulfur. EPA is requiring they lowered the amount each year.
2. EPA requiring ethanol such as E-10 and E-15 later this year and has a odd smell which can be worse by the conversion in the CAT. Review a thread on this forum about E-10 and you see it causes engine to run 10% plus leaner then normal gas which effects the content and smell of exhaust as it passes through CATs
3. The fuel trims are off, either too rich or lean again gets effected passing through the CATs and can be bettered by tuning the PCM for the car's makeup, gas used, elevation, weather and your drive style.
GM cannot control gas makeup and in their warranty states that, they do try and steer car owners to use Topdown certified gas stations that GM has tested.
List of them is on a thread on this forum