This is happening because the scale tools center is placed between the 4 verts you have selected. It automatically averages the verts locations, and places the tool (move/rotate/scale) at the center.
To achieve what your after simply select the verts. Then hit the 'insert' key on your keyboard. Then hold down the 'x' and drag the manipulator to the center, and it will snap to the center line (grid). Now hit 'insert' again and carry on with the scale operation.
Though this is more steps than just scaling as normal and then moving the verts inward.
But to answer your question, Yes this is usual behavior.
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