The placebo effect could explain your self-assessed improvement. In simple terms, just having surgery made you feel better. Once you feel better you start to do more. You may have had the strength to do those things before surgery but held back because of pain or fear of pain.
It's also possible the tests given don't measure things that changed after the operation. There are a variety of tests available to measure results in patients treated for cervical myelopathy. Some go just on the basis of the patient's symptoms. Others look at the ability to walk, work, climb stairs, or use a knife and fork.
Compare the tests done and the improvements you notice. Do they match? Does it seem like the tests were measuring the very things you've seen changes in? If yes, wait six months and take the tests again. You may see a difference with a little more time.