This is an old bug going way back.
Create a new document in INCHES. Import a DXF made with inches and all will be well. The file will import to scale.
Now, create a new document in PIXELS. Save it somewhere and close it. Then import that same DXF file. Even if you choose inches in the import settings the scale will be wrong because Illustrator is still thinking in pixels.
If you want an imported DXF's scale to be correct make sure your last document was created useing inches (or whatever the native unit of the DXF file is).
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