Why do I get different transparency on the site using css and in GIMP?

Question

css

There is a photo on the site with an rgba(10, 10, 10, .6) element overlaid on top of it . I want to get rid of an additional element by fitting the photo to the desired view in a graphic editor.
To do this, I open the image in GIMP, on top of it I place a layer with color rgb(10, 10, 10) and opacity 60% . As a result, the result on the site and in the editor is significantly different. What could be the reason?