The four volumes you refer to were the first edition of those titles. They were later revised and expanded, so this set includes the second edition of these works. These revised expanded titles have a different copyright than the original editions, which is why dynamic pricing did not apply.

For whatever reason Zondervan appears to have repackaged the series in 2014 after the first four volumes were released in 2010-11, under a slightly different title. A little different that initial series volumes were revised before other series volumes were ever released, given a single author. Based on Mark volume, other than some formatting, not evident how much material was truly revised/expanded. Faithlife should have clearly called this out in their description. You are not the first to miss this. Still, classic Dr. Swindoll insights and useful.

Revised & Repachaged? Only revision I've noticed is in the pricing. The original price was $16.99. The revised price is $25.99. :( Wish they had repackaged in cheaper wrapping paper.....