Depending on what you mean by the average developer, 1000x is plausible. If you think in terms of reliability, it's the difference between a four nines engineer and a one nine. That guy who pushes untested code that brings the site down for a day vs. the woman who has to roll back her code once in five years because she made a fencepost error is an example of how such variances in developer "productivity" can be measured by an organization, especially one that does not consider itself a technology company.
Think of floor personnel in stores. There are some who start off several steps behind because they have a lot of sick days, come in right at the start of their shift (so end up starting "late"), and take dozens of smoke breaks throughout the day without clocking out. They may chisel a minute or two extra from every break even when the store is busy. On top of this, they may drag their way through their workday. You may need 3-5 of this person to do the job of one, then you need someone to manage them.
They're going to be significantly less productive than the scarily chipper go-getter who wants to be store director, gets in early to start work on time, skips breaks or takes them only when things are slow, is very upbeat and engages customers, and offers to take extra shifts to help out. You can leave this person alone in the store and they'll keep things moving along.
Measuring productivity is a difficult science. In some environments, it's screwing up less than other people. In some, it's a simple measure of something silly, such as function points. Scientific Management has seeded a lot of bad ideas in managers' minds about measuring the value of individual contributors.