Starting with Zen Cart® 1.5.2, additional rules have been added to your store's
stylesheet.css? The short answer is that until the customer specifically refreshes their browser or clears their browser's cache (possibly multiple times), those changes won't be pulled from your site because the browser has been told to cache that file's contents!
That's where File Auto-Versioning comes into play. You can use this plugin to add a version string to those file "assets" so that your customers' browsers are always pulling in the most recent content.