To get a stable reading, I found out that heating this tube for 20-30s was not enough. The cathode seems to take a long time to heat up and stabilize. After some trial and error I ended up at 10 minutes of pre-heating before setting the tester in action. Luckily the eTracer has a bulk test mode with a warm-up timer you can set.
Notice that both E versions have a high rp. These tubes were included in some order for free, it is possible these were outliers and thus sent as a bonus?