No 'Tanker' chapter on Metal Gear Solid 2:Sons of Liberty? How is that possible?

Okay...how weird is that I started the game (on normal) and I had no Tanker chapter? No, seriously, it went straight to Plant and Raiden, I wouldn't even know that there was that previous part if I didn't see it while looking up some weapon locations x__X How is that possible? I checked and started another game yesterday, and it opened with Tanker, as it should. Why didn't it happen in the first place? o.O