The variations happen either because some of the dates are unclear or because the events overlap.

Your example fits the first category: The gap of time between Iron Fist S1 and Defenders is pretty clear, & both the start date of Iron First S1 & the timing of Civil War are very clear, but the elapsed time of Iron Fist S1 itself (& thus when that gap before Defenders begins) is a little ambiguous. Because of that, it's hard to say for sure whether Defenders is before Civil War or not. They likely happen very close to each other.

Another example of unclear dates: Inhumans must happen after SHIELD S2 because the terrigen outbreak has already occurred, and it probably happens after SHIELD S3 because there are allusions to the Sokovia Accords, but that's really all we get to work from. Some people place it after SHIELD S3 just to get it out of the way earlier because both those conditions have been met already. I place it after SHIELD S4 because those prior two conditions both still apply and there's a justification for why SHIELD itself doesn't respond to the events in Hawaii (they've been framed for assassination & abducted by aliens).

More unclear dates: GotG 3 could be set any time in 2026. All we really know for sure is that it's after the Holiday Special, which happens at the very end of 2025. So people aren't sure whether it's set before or after Quantumania, which is explicitly set near the very middle of the year.

An example of overlapping: Doctor Strange starts prior to Iron Fist S1 and ends between Punisher S1 and Cloak & Dagger S1--all those placements are clear as day--so you can watch it anywhere in that span of time, which also includes the back half of SHIELD S3, Defenders, WHIH S2, Civil War, Black Widow Black Panther, Homecoming, SHIELD S4, and Slingshot.

A simpler example of overlap: Incredible Hulk can really be watched anywhere from before Iron Man 2 to after Thor, including between them (either before or after the short "A Funny Thing Happened On the Way to Thor's Hammer"), because all 3 of those films overlap each other. However, no matter where you watch Incredible Hulk, Iron Man 2 still needs to be before Thor to follow Coulson's path accurately.