(I'm here to explain things to you politely)

50% chance given to 10% of monsters killed is 5% overall increase indeed. Think of it this way: If every 10 monsters had 1 with 50% drop rate increase, and you multiplied this process by 10, now you have a total of 500% drop chance increase given to 10 monsters, for every 100 monsters. So how much increase on average each monster gets? You divide the 500% on 100 and the outcome is 5%.

  This logic is flawed because there is no factor for how long it took to defeat Monster(x).  The buff had nothing to do with the amount of mobs; it was the time defeating mobs( present perfect tense).  If it takes 10secs to defeat 100mobs then the buff is 1sec or never depending on the base factor controlled by programming that is never revealed.  The chance never lowered there and it cannot be assumed that an average existed because no one knows "the time" factor except PA Developers.  And we all know that, from ginding extnsively, there were times when this buff would last very long or not at all.   Concluding that 5% is inaccurate assuming.

No, the buff had nothing to do with time. It was purely chance-based. A chance to apply to 10% of monsters killed.

Feel free to search over the internet to find this out, as this info no longer exist in the live game, so there's no point to debate it further.

Edit: corrected an error that was a byproduct of my attempt to explain this in "KISS" style, where I ended up confusing myself.

  There is no debate; you are misinforming players.  "% of the time" was the literal explanation on the node interface.

From the update notes today:

It's 10% chance to happen. This means for every 100 monsters, it will happen 10 times. 90 monsters won't have this buff, so using the same calculation I explained to you earlier. It's 500% on 10 monsters for every 100 monsters. Which dividing that 500% on 100 monsters (to find out how much each monster gets on average) will be 5%.