Looks like the custom-<CPUs>-<MBs> name scheme is used for the N1 family custom machine types.

I recently had this question and found in the GCloud console's Create VM UI that the N1 custom machine type is the only one that maps to this naming scheme. All the other families that support custom types use their family prefix before the custom-....

I cannot find any documentation to definitively clarify the naming scheme so I can't say this is 100% the answer, but seems like it is.