Why does the default merging generate different URLs for different page types?
Does this mean it is better to disable merging?
Yes, your site will be faster if you disable merging. Alternatively, you can use Minify JS / CSS for Magento, which merges intelligently. Your Google Page Speed will decrease if you disable merging and do not use Site Optimizer. However, Google Page Speed only measures page speed. It does not measure site speed. An individually loaded page will be faster with merging enabled, but your entire site will be slower. So, if your customers visit more than one page, you want to disable merging.
Is there a way to verify merging is done properly or not?
Quick Extension Overview
Speed up your site, improve your search engine site rankings and conversion rate, and improve your user experience in the process. Minify Js / CSS extension reduces page size, bandwidth, request count, and server load without anything more than installing this easy to use and feature-full extension.