I've seen other people post those same numbers. My personal experience (which is only a few kills each on the first two wardens) has been 100% on master 1 and with 3 valor. That's <10 attempts though so I might just be lucky.
All I know is I played on Master 1 and killed the first Keywarden (how do people play on Master 1-5 and not die???) two different times and neither time got a key or any legendary for that matter. Three stack of valor both times.