Ever shot a gun? Done so a lot? Licensed to carry concealed? Carry a lot?
I carry on my hip at between 4-5 o'clock, and it is not a small or light gun (4" aluminum frame M1911 + spare magazine on other side), so I don't exactly forget that I'm carrying it, but on the other hand the fact that I'm carrying doesn't register strongly---mostly, it's about keeping my pants on ^_^. I notice that the TSA's Instagram page shows small guns (smaller than mine) with one possible exception, and states all but the derringers were found in carry on bags.
Carrying is something you get accustomed to, and a lot of people get complacent. Massad Ayoob in his safety video talks about it becoming like "Fluffy", your pet dog or whatever.
With 43 states as of today (Illinois) having de jure or de facto shall issue regimes (plus much upstate New York, rural California and Massachusetts (!), and Rhode Island is confused), well more than 2/3rds of the population can legally carry concealed. As of last summer Clayton Cramer and the GAO independently estimated 7.5 and 8 million people have concealed carry licenses. That number is skyrocketing based on all reports, Vermont, Alaska, Arizona, Wyoming and as of this month Arkansas don't even require a license, and Illinois will be adding upwards of 10 million citizens eligible.
Yow, now 8 out of the top 10 states by population, all but California (1) and New York (3), all but 5 out of the top 20.
So, yeah, that many people, I can believe that much forgetfulness. Also, a lot of people forget spare magazines (with or without ammo), which are also in the same category.