It is because Paradox doesn't properly differentiate between percentages and percentage points. (This is also why many of the "percentage" values in EU4 don't combine the way you'd expect them to, as they are actually percentage points, not percentages.) The number of percentage points you have in a plot influences the MTTH of the plot firing, depending on certain breakpoints. If the pp are between 100 and 200, the plot has a MTTH of one year, and I believe it halves that for every additional 100 pp.Plot assassinations in general were always weird to me. Why do they go over 100%? IMO they should cap at 100% with 0 chance of being found out but make this difficult to achieve. From 1-49% lower chance of success but also a higher chance of being discovered and from 50%-99% there should be a lower chance to be discovered but a higher chance of success naturally (this all barring those annoying drunk blabbermouths). The fact that it's easier for me to pull off a plot with 20+ people as opposed to 1-2 with higher plot power always seemed ridiculous. It tempts me to want to kill every single person involved afterwards just to keep them all quiet.