This is once again making me very interested in reading the fan sequel to this book that I liked so much--because I suspect the author of that book ran into the exact same chronology problem, and that it informed some of her decisions on how to treat the secondary characters, who the book follows almost entirely. Lavinia in particular!

I suppose if I had to Watson a theory, as opposed to going all Doylist with "Gosh, the author didn't really care about timelines much on this", I would say that Lottie is dealing with stunted emotional maturity because of her emotionally neglected and disrupted early childhood, with a side order of acting deliberately as The Baby to get attention in a group environment where focused attention is hard to come by. But it's very much a bandaid over the plot hole. Character hole?