Why would a layer 3 switch ever need more switching capacity than all ports full-duplex?

Question

I've read a few different places about switching capacity etc. From these items I've learned that in order for a switch to be non-blocking, the switching capacity of said switch must be double the number of ports.

However, what isn't addressed in the items I've read is why a switch might have MORE switching capacity than double the number of ports. Take this switch, for example. At its maximum configuration, it would have 24 gigabit ethernet ports, and two 10 gigabit ports (I'm not sure what Mg is, but I believe XFP is 10 gigabit).

In that configuration, wouldn't the maximum switching capacity required be 24*2 + 2*10*2 = 88Gbps? Yet the specs claim 128Gbps. Why would they need that? Does it relate to stacking? If the user wasn't planning on stacking, would any capacity beyond 88Gbps ever possibly be used?

Answer 1

If we assume that the two dedicated stacking ports are 10Gbps, the math works out exactly.

24*2 + 4*10*2 = 128Gbps