My understanding (which I accept is limited!) is this.
In a traditional mash tun the mash water is evenly distributed through the grain. In the Brewzilla (and other similar single vessel breweries) there is part of the vessel under the false bottom which will always contain liquid that is not in contact with the grain. True enough that the recirculation will mean that the full volume of water is passed through the grain bed over time, but at any given time the space beneath the false bottom is full of water/wort that is not in contact with the grain, which will reduce the distribution of water in the grain bed. As I understand it, this uneven distribution of water in the vessel means that you cannot get the full grain bed wet when you mash in unless you increase the mash water volume from the start. In fact, I have had this myself where the grain is still dry and dusty when I've mashed in using a mash water volume from the original recipe without adjustment. You can address this slightly by running the recirculation a little during mash in to wet the top of the grain bed, but even this isn't enough in my experience - you have to add extra water. There is an amusing video on Youtube of a Northern Irish brewer cursing when his mash is like concrete when he mashes in on a Brewzilla (I forget his name - Banana Brewing or something?) and the root cause I think was not adding extra water for the mash.