Ian,
You ask "Am I getting confused here?" The answer is "yes". Unless you have the auto-exposure metering locked to the AF point - which you aren't if you're using evaluative - then the AF point used has no relationship to the exposure you achieve. You also say you try and meter off something grey. If you're using evaluative metering, then unless you're filling the frame with a mid grey subject, then it will have very little impact on the exposure. If you're trying to meter from a grey part of the scene, you would have to switch to spot metering and lock the exposure to those settings before recomposing the image as desired.
The problem you describe with the stream and the trees is due to the limitations of the dynamic range of your sensor; it wasn't much better with film, and slide film was/is worse than current dSLR. You have a range of options, none of which are much to do with the metering mode you've used on your camera:
1) Use a neutral density filter to cover the brighter parts of the scene, or
2) Take two images (on a tripod) and blend them together in photoshop, or
3) Take one image in raw format, process the raw image twice to get two different exposures and blend the two immages in photoshop (not as much lattitude as option 2, but better than nothing. Or,
4) Adjust different parts of your single jpeg selectively in photoshop.
Hope this helps.
Duncan
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