Disposal patterns like LibGDX's in languages with GCs often have to do with releasing un-managed resources in addition to (or instead of) managed memory. This true with C#'s
IDisposable interface, and based on the documentation LibGDX's pattern serves a similar purpose.
This means that simply looking at private working-set CPU-side RAM use may not be an accurate way to tell if disposing of the resource is "working as intended" or not. For example, if a texture isn't caching its image data on the CPU (expecting it to be on the GPU), then disposing it will not impact CPU-side memory appreciably at all.
LibGDX's source code is on GitHub, so you can take a look at the specific implementations of the dispose method. For example, texture's or pixmap's.