Playing around with SVG a bit. Here is a visualization of all the words in "The Hacker Crackdown", "Huckleberry Finn", and "Middlemarch":
The x-axis is the average position that the word occurs in a sentence, (so that words that typically begin sentences are farther to the left, and words that occur late, and in long sentences occur farther to the right) and the y-axis is log frequency.
Full svg files and larger pngs and the perl (sorry!) script that made them all are in this directory.
Ultimately it's just because Middlemarch is a longer book.
I cut off at the bottom all words that occur less often than 1 in 20,000 times; in the top two graphs, the total word count of the book is something ilke 100,000 words, so the rarest words you're seeing occur about 5 times. Log of five is significantly different from, say, 6 or 7, enough that you see banding. With Middlemarch the 1/20,000 mark is up around 16 I think, so the bands blur together.