How come [Cisco's] vendor documentation often doesn't include the preamble and interframe gap when calculating ethernet overhead?

I get that the preamble and the interframe gap aren't actually fields in Ethernet's layer 2 header, but they still take up bandwidth, so it seems nonsensical to exclude them from bandwidth calculations. I'm specifically thinking of VoIP - Cisco claims that a G.711 call will take-up 87kbps of data when you include RTP, UDP, IP, and Ethernet overhead. However, when you factor in the preamble and interframe gap, that number actually becomes 95kbps.