Well, that is interesting. For some colors (e.g. ff6666) I get the same value but for some others, it seems to depend on the application. For example, the ffcc66 color appears as ffcd66 in Firefox and Chrome but as ffcc66 in Gwenview and Gimp. And it doesn't seem to make a difference which color picker I use (tried Gimp, Plasma color picker, and the Firefox dev-tools).

It looks like the browsers interpret the images differently.