Damaged sectors in CDSpeed's scandisc usually represent uncorrectable C2 errors. All CD's (audio, VCD, data) implement C1 and C2 levels of error correction, but data CD's include an additional level of error detection and correction beyond the C1 and C2 levels designed to catch any errors that slip through those first two levels. The fact that you can still extract your files successfully even though you are seeing (uncorrectable) C2 errors is due to that additional layer of error detection and correction code.
As to why the sectors show up as sometimes damaged and sometimes not, that is probably caused by the statistical nature of the read errors. Errors on a disc are due to a combination of the writing process, the media quality and the reading process. Every time you read the disc, the reading conditions vary slightly. So in a marginal section of the disc where pits and lands are on the edge of readability, sometimes you may be able to read with errors, sometimes you may not.
Of course, if no combination of writing, media and reading can give you damage-free discs in scan disc, you may have a problem with your drive.
By the way, slower burning isn't always better burning, particularly with newer drives and media that are designed and tuned for high-speed burning.