My son keeps telling me about some study that found that optimists fall further behind than pessimists, but it wasn't what they expected. They expected (as a made up example) that if it required 30 days, pessimists would estimate 40 and perhaps come in on time and optimists would estimate 20 and not make it. Reality was more like it would require 40 days and pessimists would estimate 30 and optimists would estimate 20. They both got it wrong, optimists just got it more wrong.

Another issue: It is really common for people to "take some well deserved time off!" when they finish some piece of the project earlier than anticipated, thus flushing away time that could have helped out on parts that will take longer than expected.

Cuz: Humans.