dump and the tk50

[Richard Korry]

I want to dump to a TK50. The Decstation 21/3100 Operator's Guide
says that there are 600 ft of tape with 6667 BPI = 131 Mb per cartridge 
(unformatted) and the piece of paper that comes with the 
cartridge says 90 Mb (formatted). Ok, so far so good. I type the following to
dump the file system:
% /etc/dump 9dsu 6666 600 [more stuff]
which says density = 6666bpi, size = 600 ft and the dump says:
  [stuff deleted]
  DUMP: Estimates based on 600 feet of tape at a density of 6660 BPI...
  DUMP: This dump will occupy 687 (10240 byte) blocks on 0.21 tape(s).
  [stuff deleted]
Now 687 * 10240 = 7Mb. If this is .21 tapes then dump thinks that it holds
33Mb, not 90Mb. What's up?