As in the image above, when i extruded, those meshes are not the same thickness.
And when i snapped to the vertex on the right, it made weird thing on the left.
It could be that the pivot point wasn't in the exact center when scaling, or that the walls are slightly different distances from the center. To fix this you could select only one side (the front or the back) of the side wall and snap that instead of snapping it all at the same time.
Also check the scale values on your object. In your image it looks like the Y scale value is 1.2 while the X and Z are both 1. Scale inconsistencies can affect a lot in Blender like modifier behavior, constraints, and tool operations - as you've discovered with Extruding inconsistent thicknesses.
The thing to remember is that if you're getting funky behavior, first thing I check is scale values of the object. The solution is to "apply the scale values" with the CTRL + A hotkey and choosing scale. This will leave your object the same proportions while setting the scale values all to 1.0. Creating a new default state.