It's not even about some "bull reason." It's quite simply that he had a different haircut when he was younger. Have you had the same haircut your entire life?
But for the second half of the game through the end, he has the haircut you like, so what's the issue?
Not only that it's not just a different style of hair its literally just longer hair.
Whoa guys let me get this straight.
When Zack was younger his hair was shorter... and when he got older and didn't cut it... his hair got longer?
No fricking way guys.