Those are the validated OEM configurations when sold by Dell with OEM RAM.  Other configurations may work just fine, and have worked fine for others like 8 x 4, it's just not officially validated to work in that configuration.  However, if you mix 2 new sticks of aftermarket RAM with 2 sticks of OEM ram, you're more likely to encounter an issue where it does not work. 
I just bought an Alienware Aurora R10 with 16 GB RAM (DDR4 XMP Dual Channel at 3200 MHz, according to the packing slip).  When I opened up the case, it appears to be two 8 GB sticks, with two empty slots.  The two full slots alternate with the two empty ones.   I figured I'd add another 16 GB (two more 8 GB sticks).  However, when I went here and looked at the "Available Memory configuration matrix", it didn't show putting 8 GB in each of the four slots in the table? Is this table meant to be "here are examples of memory combinations" or is this table meant to be "these are the only valid combinations, and nothing else will work"?
Aurora R10, four 8 GB memory sticks?