Both doses provide the same level of protection against pregnancy, but you might be prescribed the lower dose if you’re sensitive to the hormones in Kelnor. A lower dose is less likely to cause an adverse reaction.
Also, lower doses can cause fewer side effects than higher doses, but give the same level of protection against pregnancy. For this reason, oftentimes you might be offered the lower dose even if you’re not sensitive to the hormones. The higher dose is generally used if you need higher levels of hormone to help with PMS symptoms.