Yeah, I can see your point, but I think an argument could be made either way. I think the problem with your idea is that it falls apart for the common case: single component selection.
Consider this: let’s say that the styles popup did a “merge” of the styles you’d selected in the UI and the styles that already existed on the selection.
Now, you select a component that has a style, let’s say, it has margin set on it. You decide that instead of margin, you want padding. So you remove margin in the UI and add padding. Now you hit OK. The style editor does a “merge” of the style you’d designed and the existing object. Now you have margin and padding on your style object.
Pretty sure in that case you’d also be filing a bug report.