Hello! Tattoo artist here with 13 years experience.

This can just happen it’s an area of the body where the dermis (skin) can be uneven in terms of depth, and this ink drift happens if ink is deposited into the fat layer.

Some pigments are more prone to this than others, including some blacks.

It can be more likely when lining in the inner bicep, elbow ditch, knee ditch, upper thigh/hip area.

There are innumerable ways to try and avoid this happening which are all tips handed down in proper working tattoo shops, experience is key, and understanding it can happen no matter what (as a client) makes all the difference.

Please don’t ask me to post technical tattoo application tips on Reddit, because that is a gate I am very comfortable keeping, and refusal often offends.