If a 10 day task takes longer than estimated, and there was nothing disastrous that happened, it’s natural to estimate a bit longer, say 12 days, next time a similar task needs doing. Months later, if this task is late again, the estimate will be increased a bit more to 15 days. Over a few years, a common task can easily creep to longer and longer durations, 30 days. The task appears to require 30 days because it just barely gets done on time.
It’s the same task. How can a 10 day task that is scheduled for 30 days take 30 days? What changed? There are 20 more days to complete the task.