Why does some carpal tunnel numbness and tingling go away for many people after surgery, but for some symptoms persist?

Will my sensation come back or be normal after surgery?

While the goal of carpal tunnel surgery is to relieve the pressure on the nerve not everyone will respond the same to surgery

Some patients will have immediate return of sensation while some will take longer. Some will notice an improvement right away but still feel tingling and will describe this as "numb" The return of sensation is dependent on many factors including age, general health, duration of symptoms, circulation and the actual mechanical severity of compression. This video shows some drawings that may help to explain why.

In very severe cases while decompressing the nerve stops the carpal tunnel syndrome from getting worse, full recovery of sensation may not be possible. Often this is seen in patients who have muscle wasting noted prior to surgery and in those with longstanding complete numbness and elevated two-point discrimination. Of course there are many in these categories that improve despite having very severe cases.

Having a severe case where you are not sure if you'd have full recovery is not a reason to put off surgery, as progression is likely if nothing is done.

How about my strength?

This is a very difficult question as there are many reasons why a hand with carpal tunnel may not feel as strong. It may be that the decreased sensation in the fingers prevents someone from knowing how tight to hold and object and that object is dropped more easily. With return of sensation or even a slight improvement in sensation, dropping objects becomes less of a problem. Some severe cases of Carpal Tunnel can be associated with atrophy in the muscles of the hand. In some severe cases, this muscle will never fully recover. However despite loss of muscle, function can still be preserved. In very severe cases a suregon may recommend a tendon or muscle transfer to improve function.

THEN SHOULD I DO THE SURGERY IF I WON'T or MAY NOT GET BETTER?

Holding off on surgery because of fear of not getting better is s sure way to make things worse over time.
In fact it may be a so called self fulfilling action.

 In other words the longer you wait because you fear you may not get better, the more likely that you won't get better.  Even in severe cases where your doctor warns you that there is a slim chance of improvement in your numbness or weakness, just getting rid of the pain that comes from very severe nerve compression in very severe chronic "neuropathic" cases of carpal tunnel  usually is worth it.