The amount of memory available is linked to the number of memory controllers - the full Navi 10 chip has four 64 bit controllers (or 4 pairs of eight 32 bit ones, depending on how you want to look at it).
The RX 5600 XT only has 3 functional controllers; the other one has been disabled either due to defects during the manufacturing process or due a design choice, to make the product fit its market sector better.
Since each controller is 64 bits wide, the attached memory modules cannot exceed 64 bits in total. All manufacturers of GDDR6 produce 32 bit modules and the ones used on the current Navi range are 256 Mbits in size. So each controllers manages two modules, which equates to 2 GB in total per controller.
The likes of Samsung does product 512 Mbit modules, so in theory the RX 5700 XT can have up to 16 GB of GDDR6 but for cost reasons in the desktop market, AMD uses 256 Mbit ones:
RX 5700 XT = 4 controllers = 4 x 2 x 256 Mbit x 32 bit = 8192 MB or 8 GB
RX 5600 XT = 3 controllers = 3 x 2 x 256 Mbit x 32 bit = 6144 MB or 6 GB
