Correct regarding unpredictability. In training you are in position and can execute the ideal form.
In a match, if you are not in position, your body compensates for it. This is a good thing. Humans are adaptable, we didn't evolve to repeat the same identical actions every time. In nature this would lead to injuries. Instead we adjust to the circumstances. Do not try to consciously correct your form - think about the ball you want to produce and being in position for it. If you try to consciously correct your form during a game, you will overload your mind and make things worse.
You will perform better if you are able to execute something closer to what you practise. You can do this by being in better position. You will never be in as good position in matches compared to training, but there are things you can do improve it. You need to figure out what to improve to make your in-match position better. It could be any combination of the following:
- Footwork. Are you simply not fast enough? If so, you may want to either slow the game down to give yourself time to recover and move. Not in the habit of moving? This just requires training, especially if you have a sedentary job, and also improving posture and balance.
- Not getting into a neutral position after executing the previous shot. This is also a habit even pro players fail at time to time.
- Awareness of where the next ball is going. Are you watching the opponent's bat? Are you moving as soon as the ball leaves the bat, or are you waiting too long? A lot of players wait until the ball is close to the net before moving - this is too late. Most players 'anticipate', which means they will be in position some of the time, when the ball is predictable, but be disastrously out of position the rest of the time, when the opponent is deceptive or unpredictable - this is why pivoting to play a FH from the BH won't work against a lot of opponents. Unfortunately even some coaches teach to anticipate the ball, so their students never develop proper situational awareness. This can take a lot of training to develop.
- Does your training put you in match-like conditions? In training you should be making an appropriate level of mistakes for your level. If most things you do result in a mistake, it's too hard. Likewise, if you don't make any mistakes, then it's too easy. You need to be within your range of proximal development. It needs to be challenging enough to make some mistakes.
- Regarding the last two points, you should add random play to your training. This is where you and your partner start a rally and are allowed to place the ball anywhere, at a slow pace. The goal isn't to beat your partner with power, but with deception. You will both need to be very focused to avoid the trap of anticipation and fall victim to the others' deception.