Why the 'number of photos left' is lower than actually how much can fit on card?

I have a Nikon D200, and i'm using an 8GB CF card, and it says "886" for the photos left. But I recently did a shoot, and I got well over 1, 200 photos on the card, and it still said "308" left.

Does anyone know why this is?
Is there anything I can do so I can get a more accurate number of photos left I actually have?

Oh yeah, and I shoot in Large JPG almost all the time.

JPG is a compressed format. You do not have a consistent percentage of compression on each image, so the camera has to guess. Sounds like you are getting quite a high compression rate.
If you want to get a more accurate image count, shoot in RAW (the files are MUCH bigger, but they have no data loss, like you get in JPG).

Make sure you are formatting the card in the camera once you download and backup your photos.

I'm not sure you'll be able to get an accurate count. The degree of brightness and number of colors of a subject you're photographing influences file size. For example, if you shoot a subject in black and while, the file size will be smaller than if you photographed the same subject in color.

You should occasionally format your card. Formatting recreates the file system on the card and may give you a more accurate count.

btw, it's so much better to have space left for additional images, than run out of space on a memory card.