Smart isn't foolproof, in that the technology has lots of false negatives, but history has proven that it rarely has false positives. In other words:
A failing hard drive might not have any indicators in the SMART readings that it is about to or is failing.
However...
Statistics show that when SMART attributes show that there are problems, you can bet serious money that the drive is really going to fail soon, if it hasn't already.
In other other words:
If a drive shows no impending failures after reading SMART attributes, it really doesn't mean anything, but if you see there are SMART warning signs, plan on the drive failing very soon.
I read a huge statistical study that was posted on slashdot last year, and these were the conclusions.
So you were the victim of a SMART false negative, which is common and means nothing.
-Llama