It's probably because when you installed Pod Farm you accepted the default assignment of Pod Farm as the application for processing files of type *.l6t. So henceforth, when you double-click to open these files the system uses Pod Farm.

You can open them with Vyzex instead (which I assume was the real point of your question) by either changing the default file type/application assignment in your system, or by right-clicking on the *.l6t file and selecting the Open With option, then choose Vyzex.