This is a common question many people are confused by. Why do we commit sins in Ramadan if Shaytan is locked up?
Prophet Muhammad (PBUH) told us that the Shayteen are locked up during Ramadan in an authentic Hadith. People assume this will mean they will stop committing all sins.
This is not necessarily true because we sin sometimes due to our own desires. Until we learn how to tame these desires the lack of presence of Shaytan will not affect it.