People at another site (don't know if allowed to post link) did a collective amount of runs. It was found that these superchest drops are not random. Instead there are around 65536 fixed patterns and some of them contain runes like Pul, Vex, Sur, Ber etc.
They also found (simply by doing insane amount of runs) how many patterns (out of 65536) contain a specific rune. For example 11 patterns out of the 65536 possible ones contain a Sur rune. Only 3 patterns (out of 65k) contain Ber and only 2 contain Lo. Also these finds are true ONLY FOR /PLAYERS 7 or 8. Lower player settings have different patterns (worse actually). So the probability of getting a Ber (per run and assuming two bonfires=6 superchests) is 65536/(3*6)=3641 runs. Probability of getting a Sur is 65536/(11*6)=993 runs. You can also tell if someone cheats at LK (drops a rune among the drops and claims he got it there) by checking if an actual pattern contains that rune. Also in there are 72 patterns that contain either Pul, or Um or Mal....... Ber. So the chance to get a rune ranging from Pul to Ber per run is 65536/(72*6)=
152 runsI'll correct my mistake above

edit: it seems i can't. Also if my maths are wrong please let me know
This post was edited by Cr_VI on Nov 28 2016 02:14pm