That's because Alabama never officially offered. It was a 'we'd like to have you, but we will be completely honest in that we are waiting to see what Player A and Player B do before we make you an official offer' type deals. Purdy didn't want to wait around and not have anywhere to go if Player A or Player B chose Alabama, so he chose Iowa State.
Meanwhile, A&M got a "silent commit" from the 4-star QB (their top choice) last week and backed off Purdy.
The narrative out of Ames is one thing. The truth is another. Don't ask how I know. I just do.