The growth in ratings does seem a little rapid, but ever since Amazon started accepting ratings without reviews, I've found my titles have all increased considerably. My most popular book was at around 75 reviews or so and is now at 225. It's easier to just rate than to write a review, and so far more people tend to do it.
Oh, wait, I just reread the first paragraph and saw the no sales line in the first paragraph. Yes, that makes the situation odder, but does that also mean no pages read? Because if the book in in KU and getting read, the ratings could be coming from KU readers.
Reviews can come from strange places, however. One of my shorter titles has four ratings and one review, all from a period of time during which it hardly ever sold. However, I was using the book as a free reader magnet for a while. (It wasn't in KU at the time.) The ratings came from people who received it as a magnet. Given what you said about giving 20-50 copies away on Apple each month for several months, it isn't inconceivable that some of those readers found their way to Amazon and rated the book.
The number is surprising, but I wouldn't worry about it. So far, there haven't been any other reports (that I know of, anyway) of large numbers of ratings appearing out of thin air. Maybe you just got lucky with those freebies.