happens online also . I hit perfect on every serve and more than half of them goes to the net

Attributes, positioning, stamina gauge, individual skills and types of shots affect the accuracy. It's all in the Top Spin Academy, otherwise you should get the hang of it after a number of mindful matches.

As for the serves, the timing of the hit should also match the aiming. In addition to all of the above. Also, the devs made it intentionally harder to hit wide serves, as the net is a bit higher on the edges.

I dont understand how the speed of the hitting bar works. When the opponent hits a slice (not fast) and I was ready to hit it, for some reason the hitting bar moves at a fast speed? How come? if I am in position and the slice is coming in slow, why would the hitting bar be moving fast?

The speed of the hitting bar is affected by the distance between you and the ball, fatigue and individual skills (in your case it could be the skill called "poisonous slices").