Why did the formatter insert blank pages?

Those blank pages are intentional and correct. Printed books are laid out as spreads, and certain pages traditionally begin on the right-hand (recto) side — chapter openers, the title page, and so on. When the previous chapter happens to end on a right-hand page, a blank left-hand page is inserted so the next chapter still opens on the right.

This is standard professional book design, not an error — flip through any published novel and you'll see the same blanks. See Why do chapters start on right-hand pages?

If you'd prefer chapters to start on either side (which removes most of these blanks and slightly reduces page count), the formatter has a setting for that.