Zaraki gave that explanation in Chapter 105, page 7. However, just a few chapters later (chapter 109, page 19) Zaraki chides Ichigo for relaxing and thus allowing his Reiatsu to soften. Enabling Zaraki to easily stab Ichigo. In short, because Tosen believed he had already defeated Hisagi and Komamura was lying in front of him. He let his guard down. Thus he was stabbed.