T11-T13 are estimates because Blizzard never revealed any numbers after introducing those difficulties. Also, they only said that now every rift (from Normal on, the lowest difficulty) will always yield a GR key, so while the percentages for rift keys are estimates, I've never heard anyone complain about the numbers in this sheet, and it's been out for a looooong time (I think those numbers were derived almost a year ago or so).
True, but it also says at bottom of page 'GR key and machine drop rate (for all difficulties) and all T11-T13 drop rates are estimates'.
Which makes me wonder if its accurate for low difficulties.
The only thing I can think of is, like others have said, that OP wasn't level 70 at the time of killing the RG. Well or playing an old version on console. But as of now, every level 70 rift will always yield a key.