Scout finds both the stillness of the courtroom and the amount of time the jury take to be very strange. The stillness, in retrospect, highlights how anxious many of the white people in the courtroom must have been. It would usually have been a foregone conclusion that a jury in 1930's Alabama would find in favour of a white man and against a black man, and the spectators in the courtroom, awaiting the verdict, would normally, therefore, be more relaxed, being fairly certain as to the verdict that would be returned. The fact that the courtroom is eerily still points to how overwhelmingly in favour of Tom Robinson the evidence really was.

The length of time that the jury take also seems strange to Scout. The evidence was so clearly in favour of Tom Robinson, and indeed Jem has already said in front of Scout that he "Don't see how any jury could convict on what we heard." Therefore, the fact that the jury takes so long doesn't make much sense at the time to Scout.

It's important to note that the stillness and the time taken by the jury only really seem strange to Scout, but probably not to the average reader. We, the readers, know just how unlikely the jury is to return a not guilty verdict in favour of a black man. We don't therefore, unlike Scout, find it strange when they don't promptly reappear to deliver that verdict.

The answer to this can be found in the later part of Chapter 21.  I think that the major thing that is strange is that the people (with the exception of the officers of the court) were not really acting as if they were in a regular kind of public place. Instead, Scout says that they were acting as if they were in a church.  She says that everyone was silent.  People would quickly hush any baby that started to cry and there was no chit chat going on among the people in the room.

But the officers of the court, the ones present- Atticus, Mr.
Gilmer, Judge Taylor sound asleep, and Bert, were the only ones
whose behavior seemed normal. I had never seen a packed courtroom so
still. Sometimes a baby would cry out fretfully, and a child would
scurry out, but the grown people sat as if they were in church. In the
balcony, the Negroes sat and stood around us with biblical patience.

In chapter 21, the children arrive back at the courthouse following the jury deliberation to hear the verdict of Tom Robinson's trial. When Scout takes her seat in the Negro balcony, she experiences a strange feeling. She describes the strange feeling by saying,

It was not unlike one I had last winter, and I shivered, though the night was hot. (Lee, 214)

Scout goes on to mention that the courtroom seems exactly the same as the "cold February morning" when the mockingbirds were still. Even though the courtroom is packed, Scout feels like it is a deserted, empty street. She experiences the atmosphere change to that of a winter day and says that Heck Tate "might have been wearing his high boots and lumber jacket."

When Heck Tate brings Tom Robinson back into the courtroom to hear the final verdict, Scout mentions that the courtroom takes on a "dreamlike quality." When the jury returns, Scout sees them moving like "underwater swimmers," and Judge Taylor's voice seems far away and tiny. Scout then notices that the jurors will not look Tom Robinson in the eye and compares the atmosphere in the court to the feeling of watching Atticus raise his rifle and pull the trigger, knowing that the gun is empty.

Scout, Jem, and Dill then listen as Judge Taylor proceeds to read the guilty verdict. Scout's description of the changing atmosphere and dreamlike quality builds suspense and contributes to the mood of the dramatic scene.