What factors affect creatinine levels between dialysis?

Several factors. : Simply put, creatinine is produced by muscles, enters blood and other body fluid spaces, and ultimately is removed by kidneys. Your cr level is dependent on all these things (how much your muscles release, how much any residual kidney function removes, and how much is "diluted" by excess body fluid). Dialysis removes cr, so generally the level has less clinical utility in dialysis patients.

Muscle mass: Creatinine is produced by muscle and removed by the kidneys. The exact level will depend on how muscular is the patient( more muscle means higher creatinine) and the amount of remaining function in the kidneys.