Pontus said in one stream that the percentage is higher then 10% if I remember correctly.
For the sake of the argument say it is at 15%. That means, getting no exp. weapon from 10 rivals in a row is at 19.69%, so almost every fifth player will have that happening. Getting no weapon from 20 rivals in a row is at 3.88%, so about every 25th player will have that happening.
If it’s at 10% exactly, the probability for 10 rivals not giving any exp. weapon is at 34.87% (more then every third players will have that experience) and for 20 rivals in a row it’s at 12.16% (almost every 8th player will have that happening).
As you see only a 5% change in chance has a drastic probability change for getting no exp. weapon from a lot of rivals in a row. This is why I propose of having a guaranteed drop every 10 rivals when the player had no drop for 9 rivals in a row. But then the devs would need to track that in the save file.