The way I look at it is the letters between JA and Cassandra were on-going conversations and as such I think the style is natural. I think the quick dashing from one subject to another seems abrupt to us because we are not privy to the entire conversation. There is no reason for JA to preface every comment with what was said before and by two sisters without a great deal of fortune it might have been thought a waste of paper to do so. JA tried to utilize all the blank space on the pages of her letters and of course the recipient had to pay the postage upon receiving it which would be higher the more paper used. I have linked to a post by JulieW from the 2007 Letters (29 – 38) group read with a facsimile of Ltr. 38 which shows JA writing between the lines. Also think of Jane Fairfax’s “crossed” letters in Emma. So it is not the gentle absentmindedness of Miss Bates but the closeness and rapport between two sisters who knew each other too well to require repeating background information in their letters. (:D)