Intensive linguistics

Today, my world was thrown upside-down when a coworker informed me that the saying, "for all intents and purposes" is not actually pronounced/spelled "for all intensive purposes."

Ummmm, did anyone else know that? I know I'm only asking for the inevitable here, but am I just an idiot? I was so befuddled that we went online to check and found fairly conclusive evidence at this site. What the hell?!??!? How come no one has ever corrected me? Did people read stuff of mine and snicker whenever I wrote "intensive purposes", just like I snicker when people misuse affect/effect? I demand satisfaction!