It’s called water or hull line and occurs when there is a large sudden change in layer times where the underlying layer perimeter area(s) contracted before the next perimeter layer above is printed. It’s a common occurrence and is primarily caused by the part design in connection on how FFF/FDM printing works. Amount of walls, infill type, filament material type, temperature deltas, speeds, wall to infill overlap,…etc. are also factors on the perimeter size differences in these areas. Certain slicer settings like printing infill first before walls can help in reducing or even eliminating these water lines but can cause issues like wall to infill separations. Better way is to design the part with this in mind and use features that hide these steps by making it look like an intended design feature or smooth out the transitions by using internal fillets on the adjacent wall 
Do a search on “Benchy hull line” and you may encounter some interesting conversations, possible solutions and also some head bashing on that issue.