It would be hard to say exactly what might be causing the browser(s) to cache any content without knowing what exactly you’re seeing being cached.
Browsers are getting more and more aggressive with caching content. There’s been a browser war going on for years. Each browser wanting to assertion prize of being the fastest of the bunch.
One of the easiest way to give the impression that the browser is faster is to cache things that don’t have any specific cache instructions in the HTTP headers.
For most users on a shared hosting environment, the best way to add caching directives to the header is with the htaccess file.
On this post I give some samples that you can use as a starting point. You’ll notice that the sample even caches HTML for 10 minutes. This should ”force” the browser to reload the HTML after 10minutes. That gives your user a faster experience if they bounce back and forth between pages.