Question: How to explain this behavior of seq()?

The following works as intended:

diff(x^7, x$i):
seq(%, i=0..7);


Combining the two commands into one, however, does not work:

seq(diff(x^7, x$i), i=0..7);

          Error, invalid input: diff expects 2 or more arguments, but received 1

How does one explane this?  I was unable to find the reason by looking at the help page for seq().

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