Why does Scout think the trial is a "grave disappointment" in Harper Lee's To Kill a Mockingbird?

Just after Sheriff Heck Tate's testimony in Chapter 17 of Harper Lee's To Kill a Mockingbird, Scout reflects the following to herself:

So far, things were utterly dull: nobody had thundered, there were no arguments between opposing counsel, there was no drama; a grave disappointment to all present, it seemed. (Ch. 17)

In other words, at this early stage in the trial, being young and naive, Scout thinks the trial is proving to be boring just because "there was no drama," meaning that no one was yelling, raising objections, or arguing. However, being young and naive, Scout has just missed some significant details in Sheriff Tate's testimony that paint the unfairness of the entire trial, details that Jem and the rest of the spectators, especially the African Americans, have caught.

The first significant detail in Sheriff Tate's testimony is his statement that a doctor had not been summoned to examine Mayella on the night of the crime. Instead, Sheriff Tate saw Mayella's bruises and assumed that, as he declares to Atticus on the witness stand, "Something sho' happened, it was obvious." However, the problem is that Tom Robinson is being brought to trial on a rape charge, not on an assault and battery charge. A doctor's examination would have been crucial for proving a rape actually took place that night, and a person cannot be tried for a crime without concrete evidence that the crime actually took place. Jem clearly understands the significance of Sheriff Tate's testimony that a doctor had not been summoned as Scout notes his "hand, which was resting on the balcony rail, tightened around it," and "He drew in his breath suddenly." Scout, being too young to understand why a doctor should have been called, fails to recognize the importance of Sheriff Tate's statement.

The second significant detail in Sheriff Tate's testimony is that he states, after having to think about it for a bit, that Mayella had been bruised in her right eye, facing her attacker. After saying this, even Sheriff Tate reacts in such a way as to show he understands the significance of his statement. Scout describes that "Mr. Tate blinked again, as if something had suddenly been made plain to him"; he then looks straight at Robinson. Scout further describes the sudden restlessness of the spectators. Particularly, she describes that the African Americans in the courtroom began whispering "among themselves." All of the tension indicates Sheriff Tate has just said something extremely crucial to the case, and Scout, being too young and uninformed, has failed to understand the significance of the tension. Later, we learn exactly why Sheriff Tate and the courtroom spectators grew tense: Robinson is crippled in his left arm and hand; therefore, it is physically impossible for him to have been able to bruise Mayella's right eye while facing her or to cause the other injuries Sheriff Tate described.

Hence, in Scout's mind, the case is progressing in a boring manner, but she only feels this way because she is too young to understand all she is hearing.