Nick, first of all great write-up, it's really nice when people take time from their schedule to write such detailed informations about how their internal systems work!
I could be wrong but from the article and the numbers you posted it seems that you do cache 89% of your db queries results so maybe this is what jldugger referred to :
504,816,843 (+170,244,740) SQL Queries (from HTTP requests alone)
5,831,683,114 (+5,418,818,063) Redis hits
Ah I see the confusion now. To clarify: we use redis for a great many things, not just serving HTTP requests. For example, we use redis lists and sets to continually recalculate mobile feeds for users - that's roughly 4 billion of the hits every day.
I could be wrong but from the article and the numbers you posted it seems that you do cache 89% of your db queries results so maybe this is what jldugger referred to : 504,816,843 (+170,244,740) SQL Queries (from HTTP requests alone) 5,831,683,114 (+5,418,818,063) Redis hits