For me the level range always seemed to be the suggested starting one, so for this Dungeon one of 6th-9th a party of 9th level characters should be reasonable going in.
Then with megamodules like
T1-4 Temple of Elemental Evil the cover says for levels 1-8 but it is really for 1st level parties and takes them to level 8. 8th level characters should not be taking on the beginning of the megamodule.
When for planning purposes I started making my own list of the modules I owned and what levels they were designed for, this discrepancy in intended level range information became apparent.
This latter style seemed to be more and more common over time, so for instance all of Pathfinder's Adventure Path modules the intro will generally say for levels 1-4 but they are for 1st level parties and will advance them over the course of the adventure to level 4 with level appropriate encounters laid out in expected order of encountering them. Most of the WotC hardcover 5e modules are big megamodules that are similar, designed for a specific starting level and going up to a targeted level for the climax by design.
1e accommodated a bunch of level discrepancy so a party might very well have 6th and 9th level characters in the same party just from class xp discrepancy, a culture of characters being used in different games and earning different amounts of xp, multiclassing, xp bonuses from stats, PCs starting from level 1 at different times, energy drain, and magic items that can grant levels. In 2e individual xp awards based on class or individual actions could vary widely.
With 3e the discrepancy in expected character level in a party was significantly reduced with every class having the same xp chart, xp generally being based on the party overcoming challenges, and classes being designed to be combat balanced at equal levels unlike in prior editions. Xp discrepancy and level discrepancy could be there from item creation, favored racial class mechanics, energy drain, and level adjustment races, but these were significantly less than prior editions and the xp discrepancy mechanics were eventually phased out as the editions continued on.
At various points in the post AD&D editions the CR math was also designed to be tighter to a specified level of challenge and so it is arguable the game became less forgiving of level discrepancies in the party. The tying of xp to encounter challenge and a single xp chart also made it easier to design challenges of a targeted toughness to a specific expected level in a module. Both of these factors would make the design of a module for a single specified level more functional and favored the latter style of level range designation.