Consider the definition:

 Clear[fun]
 fun[a_ + b_] := fun[a] + fun[b];
 fun[a+b+c]
 (*fun[a] + fun[b] + fun[c]*)

This works as expected. However, if we use a `BlankSequence` on "b":

 Clear[fun]
 fun[a_ + b__] := fun[a] + fun[b];
 fun[a + b + c]
 (*fun[a] + fun[b, c]*)

it seems that `BlankSequence` eliminates the `Plus` in "b+c" and replaces it with `Sequence`.
Has anybody a good explanation for this behaviour?