Actually we do it for reason, actually this approach improves performance a lot. 

UNA load 2 combined CSS files on any page. 1st one is big common CSS file which is the same on all pages, 2nd is small with custom styles for each page. Since 1st file is the same on all pages, it works very fast since it's cached. If we would load only necessary CSS files for each page then CSS files would be different for each page and cache can't be used effectively, since new file is loaded each time. So we pack quite a lot of CSS in a 1st big file, but since it's should be cached for 30 days by default, it should take 1-2 seconds more only first time, but then next 30 days it will save a lot of time and traffic.