As keratoconus progresses, the cone in your cornea continues to steepen. Each time it does, the axis and strength of your astigmatism shift, meaning the glasses that corrected your vision a few months ago may no longer match your corneal shape.
In early keratoconus, these shifts can look like ordinary prescription updates. Your prescriber increases the cylinder correction and adjusts the axis, and the new glasses seem to help for a while. As the disease advances, each new prescription corrects less of the distortion, and the need for updates comes sooner.