size/lgk values i am using default only in both the cases

+1 on what Ben said ... it looks like the sketch results can change based on segment and historical configuration. For the HLL sketches I found that in one particular case a value of 18 produced stable and accurate results (compared with an exact count distinct), whereas 16 or lower did not. I believe the default value is 12. So if you have the resource (memory/cpu) available you might want to try upping the value to see if it improves the results.