Perhaps shorter logs give more variance. I have an average log length of 41 words, and a similarity score of 31%. Although I sometimes write full length descriptions, I often post logs such as "Another quick find here - TFTC" (I'm not saying I should, and I will write more for a good cache, or more effort than usual needed to find it). Personally I'd rather have a higher log length and lose out on the similarity score, since I get a better badge for the log length, whereas the similarity score doesn't seem to be used in any other statistics. However with over 2000 logs, bringing the average length up is very slow!