Two things - firstly, each of the scores you see in the key findings are just the average. We have also estimated the size of the standard deviation (see table in section 2, or appendix A). So for some treatments, large uplifts are not out of the question.

Maybe more importantly - every A/B test ever run suffers from measurement error, and usually in e-commerce this error is on the scale of the effect you are trying to measure. This means that sometimes you will 'see' massive uplifts, where in actuality most of the size of the effect was due to random noise. This is kind of the curse of e-commerce : most people have enough data to say something (we are 95% sure this test was positive), but most not with any notable precision (we are 95% sure the uplift was between +8% and +9%). Basically all the stats in this analysis is trying to remove this noise, and this is what we got.