Apparently Tom's testimony comments on an event not recalled by Mayella in her testimony.
In addition to the above comment, afterward, she made some comments. Tom says she noted that she had not ever kissed anyone before so she might as well kiss a "nigger." He also commented that she said what her daddy do to her don't count.
This is a very subtle hint, but it leads the reader to believe that Bob Ewell uses his daughter. He may be the one closer to guilt of the crime at hand.
The answers to these questions can be found through a close reading of chapter 19. In this chapter, Atticus questions Tom Robinson on the stand, and he gets a chance to tell his side of the story. Mayella asked him to get something off of the dresser, and so he gets up on the chair. While he is on the chair, she grabs him around the legs; this throws him off balance, and he hops down, knocking the chair over. Then, Tom states, "she sorta jumped on me." So, she jumped on him, and hugged him around the waist, then kissed him on the side of the face. He tried to run out, but she grabbed him, so he had to push her away. At that point, Mayella's dad came in, and Tom ran away.
That is not the story that Mayella and her father tell, however. Atticus, through his questioning, makes it pretty clear that they are lying and that Tom is telling the truth. I hope that helped; good luck!
1) Mayella first lured Tom into the Ewell home because she had "somethin' for you to do in the house." She then requested that Tom stand on top of a chair and get a box down from the top of the chiffarobe. Then "she'd grabbed me round the legs." It scared Tom so bad that he hopped off the chair and turned it over.
2) When Tom jumped off the chair, Mayella "sorta jumped on me... She hugged me round the waist."
3) Then "she reached up an' kissed me 'side of th' face. She said she never kissed a grown man before an' she might as well kiss a nigger." Tom refused to kiss her back; when she blocked the door, he pushed her aside and ran out.
The answer to these questions can be found in Chapter 19. Tom Robinson is on the witness stand in this chapter and Atticus is having him tell his side of the story. Basically, he is saying that Mayella Ewell came on to him.
While he was on the chair, he says, she grabbed him around the legs. When that happened, he was so scared that he jumped down, knocking over the chair. After he did that, she "jumped on him." By this, he means that she hugged him around the waist. Finally, he says, she kissed him.