The process as of 2.6.27-rc1 was that code would be written and tested before the merge window, submitted during the merge window, and merged whenever Linus felt like it; there wasn't a process in place to submit changes intended for 2.6.27 before 2.6.26 came out. Plus, he was a big softie, some developers weren't clear on the process. Also, not just any bug fix is acceptable after the merge window; there ended up being push back on a ton of code post-merge-window for 2.6.27 that was fixing features introduced in 2.6.25 and 2.6.26.
IIRC, there was talk at the summit about going to a model where everything for 2.6.28 must be submitted before 2.6.27 came out, by way of -next. But Linus didn't want to have to deal with pull requests for 2.6.28 while he's trying to get 2.6.27 released.