What I heard was that some people have mutations that impair digestion (absorption requires some cellular machinery) or in other cases the utilization of B1.

Apparently the mechanisms for absorbing it depend on having sufficient B1, and can get "stuck" in a deficient mode if you went through a period of deficiency, which can then be "unblocked" (kickstarted) with a megadose.

Take this with a grain of salt, it's supposedly based in research, but I didn't check any papers about it yet.