LucidSW - 7:36 pm on Nov 30, 2011 (gmt 0)
Quality Score is mainly based on CTR but that's _relative_ CTR, not absolute. This is basically what Buckworks said, a correction, normalization in math speak. In other words, seasonal effects but in this case, it's positional effects.
As for conversion rates, the same is often true: your conversions are higher the higher your ad. Best way I heard it explained is "first position ad gets first crack at making the sale". I got lots of client data to show this. Since first position gets higher CTR and conversion rate, it's the best ROI. The CPC is often lower too.