This principally applies to the Stratemeyer Syndicate series like the Bobbsey Twins, Hardy Boys and Nancy Drew. Old copyrights were replaced with new ones by shortening and updating the stories. Some stories were completely rewritten even though the old titles or very similar ones and volume numbers were retained.
In the case of Nancy Drew, the first 34 books were revised from 25 chapters and roughly 214 pages and copyrights from 1930 to 1957 to stories with 20 chapters, 180 pages, and new copyrights from 1959 to 1977.
The first 37 Hardy Boys books were revised between 1959 and 1973. Some of the OT stories already had 20 chapters.
The Bobbsey Twins books were revised in 1950 (first 3) and again in 1961 (first 3 plus others). The latter revisions made the stories more mystery oriented and the titles were often modified to reflect this.
Part of the motivation to revise the Bobbsey Twins was the discovery that several volumes had not had their copyrights renewed after the initial 28-year term. Having some volumes be public domain that could be published by anyone was undesirable so the publisher suggested creating new stories with updated content for 1950.
While updating the books for new copyrights, new printing technology could be used as well. Copper electrotype plates were replaced with photo offset masters.
As the old books were products of their time in which they were created, so were the new ones. Stereotypes were no longer acceptable and written out of the stories to address some complaints. This is not the only reason for the revisions, the financial considerations of copyright and printing technology were the larger ones to a publisher like Grosset & Dunlap.
Some other series had smaller changes that can be detected. However, these abbreviations are not as frequently used on series like Kay Tracey, the Mercer Boys, and others with revised texts.