They're not. Think of it like a head & a single 8Ω cab. Also watts do not equate to the SPL. It doesn't quite work like that.
Your combo (the newer version of mine) is just the same as buying a LMIII & a 2x10 8Ω cab. It puts out the same SPL, the cab is rated at 400w & the xmax is the same & so on.
If it was a 4Ω cab, you're still gonna have a 400w cab that is still gonna give the same amount of SPL but you wouldn't be able to add another cab to it to increase that SPL.
So when looking at combos, go for the ones that are on an 8Ω load so you can add to it (as opposed to being stuck when you run out of oomph & canny add another cab).