I'm afraid I can't tell you how the figures were derived, Embraer did approval flights into LCY but don't think they had anything to do with landing performance per se I suspect they are derived from a computer model. We did have a discussion about whether a grooved rwy could ever be considered wet but didn't really get anywhere due to basically what safetypee says above. There are however many factors involved for instance the 190 can accept a higher tailwind than the 170 due , I'm told, to the tyres which have a much flatter profile on the 190 hence greater contact patch. You might be better trying to contact someone who has experience of operating you a/c type into LCY.