When new plugin version is released, it usually contains changes to CSS and JavaScript. If you use some sort of cache (plugin for cache, server side cache, CDN), and depending on your system configuration, CSS and JS files can be cached, and even with new version of the plugin installed, old files can be served from cache.
Whenever you update the plugin, make sure to clear all cached data from cache plugin, server-side cache, CDN and finally browser cache.