I'm glad I've opened up a wee debate here, although I must admit that a lot of it is over my head.
I've found the answer to my original question. Essentially, the cab alt shown on EICAS is calculated in SL ft/min whereas that shown on the vertical speed indicator is in standard ft/min. So when delta p equals zero, and the aircraft makes an emergency decent, the value shown on the EICAS cab alt will be far smaller than that of the actual a/c rate of descent
This is also true on ECAM displays.