A 45% “Top of Page” rating means that if this ad ran 100 times, it would have appeared 45 times at the top of the SERP (above the organic results) as opposed to the bottom (or pre-February 2016 — as opposed to the side). Also, having a high percent of ads in the above the SERP is very strongly correlated with high quality score.

In MySpyFu keywords, the Top of Page % appears in the group metrics and in the individual keyword metrics where we update the rankings more frequently. That metric might differ from what you see in month to month metrics. Our MySpyFu metric for Top of Page % reflects the average during the entire time we’ve been tracking the keyword(s).
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The stat used to reflect positions 1-3, but for SERPs where the first ad is at the bottom of the page, we won’t consider any ads to be “top of page.”