First of all, these episodes were NEVER 24 to 26 minutes long! Don't know where you're getting your info, but it is WRONG. The episodes on this set clock in at around 22 minutes each, which is correct. A handful of early episodes had incomplete animation that was only seen in the original airing and corrected by the original studio ever since, which is what are on these sets. For instance, the original airing of "Slave Island, Part 3" ended with the X-Men arriving at an intact X-Mansion instead of one demolished by Juggernaut. Due to animation delays/ problems with "The Unstoppable Juggernaut" episode (which was to follow chronologically), the ending was altered, since the Juggernaut episode would not be ready to air for several weeks. So "The Cure" and "Come The Apocalypse", which were ready to be aired, were shown first instead.
In fact, this series is one of the few that I would say having the original broadcast versions is not better. So many of those early episodes aired initially with incomplete special effects shots, coloring mistakes, dialogue sync errors, poor animation scenes, and so forth. They were eventually corrected in subsequent airings, but I've always feared that a studio releasing this series on DVD would give us the flawed original versions. Luckily, Disney seems to have done their homework with this one even though we were not treated to a full video/ audio restoration job. Further proof that these are not the syndicated versions - this set features the original voices of Storm and Sinister, which were re-dubbed by the replacement voice actors in reruns. BOTTOM LINE: Disney has given us the corrected versions of certain episodes that originally aired with animation errors and NOT the edited for time syndicated reruns.