There are a
lot of sources of sloppiness here, though.
The question is sloppy relative to the definitions. They asked "Just your best guess, what percent of Americans today would you say are gay or lesbian?", but the measuring stick is what portion of people identify as gay, lesbian, bisexual, or transgender. I would be a little surprised if people use exactly the same framework to decide if someone else is gay as they use to decide if they want to personally identify as gay.
There's no I-have-no-clue choice.
If you assume some fraction of all respondents are just guessing, and they know they're guessing based on a 100% scale, it would make some sense for the results to skew towards a choice given 75% of the scale.
Young people give estimates that are significantly higher, but a cursory search (not sure about the veracity) suggests that LGBT self-reports also sort very strongly by age (https://williamsinstitute.law.ucla.edu/visualization/lgbt-st...). As far as I can tell, the survey Gallup is using (on identification) doesn't provide an age breakout. I don't want to be too hyperbolic, but the change in the acceptability of identifying as LGBT over the last 70 years is pretty profound, and it seems at least plausible from the data that much of the difference in age groups (and, for that matter, partisan affiliation) could disappear if controlled for different identification rates among people's actual peer groups.
If so, the headline is more like, "Most Americans assume everyone in America is a bit more like their peers than they are."
This feels a little bit like asking one set of people "How many Americans have ever cheated on a test?" and asking another set "Have you ever cheated on a test?" and then publishing an article about how "Americans Greatly Overestimate How Many Americans Have Cheated on a Test"