Converting an NFA to regex using GNFA algorithm?
You want to know why "the others" obtained a different expression?

When contructing a regular expression there is no unique correct answer. There are usually several expressions "that make sense". In this case you use state elimination method (I learned this under the name of Brzozowski and McCluskey). Here the order of removal determines the expression found. So: what do you get when removing $q2$ first?

You can also do "clever tricks" during the construction. In your example you have $b\cup aa^*b$ which is equivalent to $a^*b$.