Acts 9:1-19 is the original story of the conversion of Saul as he traveled on the road to Damascus - this is the author of Acts (often thought to be Luke the Evangelist) explaining to Theophilus what happened and how.
Acts 22 takes place when Paul has been arrested in Jerusalem, after his journeys of conversion of the Gentiles and preaching throughout Asia. Some of the Jews of Jerusalem, who were opposed to Paul's efforts to bring Greeks and Gentiles into the new, developing body of believers in Jesus as the Christ, stirred up a mob demonstration against Paul and he was arrested by the Roman authorities. When he demonstrated his education and explained that he was a Roman citizen as well as a Jew, he was given an opportunity to address the crowd in his own defense.
And when he had given him leave, Paul, standing on the steps, motioned with his hand to the people; and when there was a great hush, he spoke to them in the Hebrew language, saying: "Brethren and fathers, hear the defense which I now make before you." (Acts 21:40-22:1)
Acts 22 draws upon his history as a strict Jew, raised with extensive knowledge of the Torah and the Jewish writings. Paul draws on this background as he explains his actions and the reasons for them, knowing that his audience was Jewish and hoping they would honor those experiences.
Acts 26 tells of Paul's appearance and speech before Roman officials including Agrippa, his wife Bernice and Festus.
I think myself fortunate that it is before you, King Agrippa, I am to make my defense today against all the accusations of the Jews, because you are especially familiar with all customs and controversies of the Jews; therefore I beg you to listen to me patiently. (Acts 26:2-3)
Paul had appealed to Ceasar for ruling regarding his arrest, which he had a right to do as a Roman citizen. The Roman territorial authorities in Jerusalem could find no reason to continue the case against him and were inclined to release him if he had not made the request to be taken to Rome. Acts 26 is Paul presenting his story to authorities who do not have the knowledge of the Jewish background but who are very familiar with the workings of gods, the ways in which power and authority may be granted, and who are curious about this new movement that is making itself known in their regions.
The differences reflect the different circumstances under which the story was retold.