More on film speeds

This is related to Bob's question below. It seems most workers rate B&W films at less than the manufacturers published speed, unless they push. Many people suggest calibrating your system using transparancy film, as exposure is so much more critical, yet I don't see people making huge changes from the manufacturers published speeds on that. Is the measurement of speed different for the different materials? Or is it that transparancies are metered more for the highlights, whereas negs are metered more for the shadows? Or is 0.1 over b+f for b&w films really a valid criteria at all? Equipment accuracy today is pretty good, yet it seems that accurate exposure is as personal now as it was 50 years ago!

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