Because we are more likely to see no big changes than to see another big change.
if the risk is usually quite low (e.g. 0.001 % per century), but sometimes jumps to a high value (e.g. 1 % per century), the cumulative risk (over all time) may still be significantly below 100 % (e.g. 90 %) if the magnitude of the jumps decreases quickly, and risk does not stay high for long.
I would call this model “transient deviation” rather than “random walk” or “regular oscillation”
Because we are more likely to see no big changes than to see another big change.
I would call this model “transient deviation” rather than “random walk” or “regular oscillation”