It's pretty common in cats for there to be a honeymoon period after treatment begins. The lower blood sugar levels allow the still viable ß cells to resume producing insulin.

After a time, however, the process that was killing them off in the first place often reappears and insulin is required again.

The biggest factor I've seen in previous cats with diabetes - dry vs wet food. Switch from free-feeding dry to measured, timed canned portions made a night/day difference.