I assume you mean her change of mind when she told her father that she would, in fact, marry Parris. Until this moment, she had refused to marry Parris, but she has now told her father that she has changed her mind.
As readers, we know that the reason she told her father that she will marry Parris is because she is lying to him. She actually plans to marry Romeo and run away with him. From the parents' perspective, though, Romeo is not in the picture, and they just see a young, confused girl. Capulet threatens to put Juliet on the streets if she does not do what she is told. When she comes back after refusing in the first place, she is apologetic and begs for forgiveness. Capulet asks where she has been and she responds:
Where I have learned me to repent the sin
Of disobedient opposition To you and your behests, and am enjoined By holy Lawrence to fall prostrate here To beg your pardon. (falls to her knees) Pardon, I beseech you! Henceforward I am ever ruled by you.
Juliet may seem sincere in her apology from her father's perspective, but reade's know she is not. She is just telling her father what he needs to hear in order for her plan to work out. Because Capulet is too self-absorbed to consider that she is anything other than genuine, he does not question her apology. In his mind, she was out of her mind to defy him in the first place, so now she has just come to her senses. To answer your question, Capulet probably should have been surprised by her quick change of mind, but he did not see it that way. He only saw things in terms of a daughter being disobedient at first and then repenting. The aspect of who she was or was not in love with was not important to him.
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