That is weird. I would think if the third book would be the one that would sell the worst, from people who star the series but stop before they get to the end.
Not so weird. For a popular mini-series, the print run is often upped for each installment. With "Mission: Gamma", the first book had the word count of two-and-a-half
(then-standard) ST novels and yet sold very well, so extra copies were ordered for each subsequent part. Meaning that Book 1, which was expensive to publish, ended up quickly falling out of print.
Some of the early duologies came out at the same time, and some trilogies came out two parts in one month and one in another. Lots of variables. Predicting print runs for the first edition of a novel must be an awful chore.
The middle volume of "Millennium" was the one I struggled with. The first book was supported by an audio novel and the final part was a great book.