I've been teaching a project management workshop, and one of the participants said something brilliant: “If you estimate in days, you'll be off by days. If you estimate in weeks, you'll be off by weeks.” If you estimate in months, you will be off by months.
Here's why. The more you can break a big task apart, the more likely you are to remember all the pieces and estimate each piece well. The less you know about a task, the more gotchas you'll encounter, and the longer the task will take. And, the bigger the task, the more likely you are to fall into student syndrome.
If you're a PM and you don't understand why your schedule is slipping, look at the general task duration. Got a lot of week-long tasks? Or multiple-week-long tasks? Those tasks are slipping, and you won't know why or by how much until the time is almost done. I bet your project will slip for a duration of several of those multi-week tasks. Replan now, breaking all those tasks into inch-pebbles. Then you'll have a much better idea of what it will really take to finish this project. And maybe, just maybe, you won't have that much of a delay, because delays of weeks are very different than delays of days.