It seems totally random, and you cannot rely on watts anymore.
So a 100 watt GAN charger might be able to deliver only 65 watts from it's main "laptop" port, but it has two other ports that can do 25 and 10 watts each. Still 100 watts in total, but your laptop will never get it's 100 watts.
Not every brand is as transparent about this, sometimes it's only visible in product marketing images instead of real specs. Real shady.
That might not necessarily be the right conclusion. My understanding is: almost all USB-C power cables you will enounter day to day support a max current of at most 3A (the most that a cable can signal support for without an emarker). That means that, technically, the highest power USB-PD profile they support is 60W (3A at 20V), and the charger should detect that and not offer the 65W profile, which requires 3.25A.
Maybe some chargers ignore that and offer it anyway, since 3.25A isn't that much more than 3A. For ones that don't and degrade to offering 60W, if a laptop strictly wants 65W, it won't charge off of them.
So it's worth acquiring a cable that specifically supports 5A to try, which is needed for every profile above 60W (and such a cable should support all profiles up to the 240W one, which is 5A*48V).
(I might be mistaken about some of that, it's just what I cobbled together while trying to figure out what chargers work with my extremely-picky-about-power lenovo x1e)
And wow, I'll keep away from Dell, thanks.
The ones I use most are 20W and 40W, just stuff I ordered from AliExpress (Baseus brand I think).
I can't find a USB-C PD adapter for a laptop that uses less than 100W. As a result, I can't charge a 65W laptop from a 65W port because the adapter doesn't even work unless the port is at least 100W.
It does not always work.