Why can the Powder be our sole nutritional source but the Bars can't?

Hey all.

I’ve been using Huel for quite some time now and it only recently occurred to me that I can swap one scoop of the Huel Powder for one Huel Bar, as their nutritional profiles are quite similar.

So, that’s my question. If the nutritional content of both products are almost evenly matched, why is it that the Bars “should not be used as your sole source of nutrition” despite being a “complete and balanced source of macronutrients and micronutrients”?