We're playing the playtest material tonight, but in looking over the provided PCs I can't figure out what the designers were thinking.
I'm playing the Fighter, and I note that my chainmail gives me an AC of 15. However, I could wear a chain shirt instead, get 14 + Dex Mod, have the same AC at 15, and be wearing light armor rather than taking all the disads of heavy armor. Also carrying a DEX-driven Light Crossbow instead of a STR-driven Heavy Crossbow. Odd.
The Thief pregen is also purposefully underarmored, wearing leather instead of a chain shirt. I presume so that he will not have an AC of 17 and outperform the Fighter