That high attenuation could easily have been caused by an exceptionally fermentable wort. Did you mash for a long time or at low temp or both? 148 for 75 minutes and you could get sown that low.
The temp is kinda high for a kolsch yeast but I don't really think it would bump the attenuation by more than a couple gravity points. You would more likely see that effect in the flavor. - fusels and fruit/bubblegum.
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