Brush line is jagged on one file and smooth in the other file. Why?

[Extremlity]

In the picture on the left is a jagged line for a reason unknown to me. On the right the image is made new file with the same dimensions, zoom, and copied layer with a brush line, and looks smooth. Why? When the zoom is 100% then see everything good in both files and properly, but as soon as zoom in, there is a problem.
Does anyone know how to fix the file from the left image to have smooth lines as in the right image, without transferring all the layers to another file?

Thanks

[JJMack]

Here is what the right one looks with an active selection captured. To me the left one looks like a screen capture of a  selection so the ants are stationary not marching.