Everything Richard has said in this thread is correct.
When using Tremote, the Thumbnail Slider setting is retrieved from the Server Library and applied to the Client display. Because the screen resolutions and DPI are different, this results in different sized thumbnails on the Client from the Server. My HTPC Server runs at 4K (3840x2160) on a 65" TV. My Workstation Client runs at 2560x1600 on a 30" display. So setting the Server to 15 thumbnails across gives 10 thumbnails across on the Client. The actual number of thumbnails across depends on the width of the Navigation Bar, and I often have a vertical Split View open, both settings that are not dependent on the Server, so I see even less usually on the Client.
Perhaps a better solution than having a than a global setting for the slider, that was applied to all Views, would be for the slider position to take into account the different resolutions on the Server and Client, so that a slider setting on the Server giving a certain number of thumbnails across gives the same number of thumbnails across on the Client, assuming other interface settings (Navigation Bar width, Split View, etc.) are the same.
I'm always changing thumbnails size on both my Client and Server, so this issue doesn't affect me much. But if I was using a SurfacePro and Tremote as a remote control for my Server, I could see how this would be annoying.
Tip: Did you know that right-clicking on the Slider maximises the view of the current thumbnail, show just that one image? Right-clicking on it again revert to the original Slider setting. Neat.