When Greasemonkey scripts run?

Recently I found one interesting thing about Greasemonkey. I was moving my scripts from JQuery to plain Javascript and changed $(document).ready() to window.onload() and my script stopped to work. After investigating a while I found that $(document).ready() is called also AFTER document is loaded and window onload obviously only when onload event is fired. This is interesting feature by itself, but I was surprised that Greasemoneky code is called only after document is loaded. Somehow I was not thinking about it this way with many many years of onload usage.

Anyway this is great, because my Greasemonkey scripts looks much cleaner without these unnecessary callbacks - just plain logic.