User blog comment:Eduardovongola/Renji Overpower/@comment-1268838-20131228183426/@comment-11051098-20131228201856

Rose had been a captain for 2 years when the TBTP mini-arc started, stated by Shinji. 9 more years passed before the hollowfication experiments. Then there's been roughly 1,5 years since FKT, so Rose have only been a captain for about 12,5 years. The 100 years in exile didn't necessarily contain that much training other than staying in shape, and learning to use their Hollow masks. As for the other Visored, we don't know how long they were captains. Shinji confirmed that there had been a rapid change of captains lately, at the time Kirio was promoted to the RG and they yet again had to find a new captain.

Shinji, Love and Kensei could possibly have had only 15-20 years in total as captains before the exile.

I must admit that I thought Kensei was stronger than what he was shown to be when he was beat up by Mask. And despite being trained by the RG, Renji is far stronger than I would think possible, easily blocking attacks of a strength that took out Kensei. Kensei might have underestimated Mask too, making him easier to defeat.

Also, we have to remember that Rose gave away the secret of how his Bankai works, and the shock of seeing Mask crush his eardrums, gave Mask an opening to take him out.