It seems that whenever STDIN is involved the results padding reverts to the BSD-style 7/8 padding. When files are given as input (excluding STDIN) the padding reflects the width of the largest count. When files are given as input and one of these is "-", the padding reverts again to the BSD 7/8 padding.
System: Arch Linux (package: core/coreutils 8.23-1)
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$ wc --version
wc (GNU coreutils) 8.23
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Written by Paul Rubin and David MacKenzie.
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Actual:
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$ echo "some text" > test.txt
$ printf %s "one\ntwo\nthree" | wc - test.txt
3 3 14 -
1 2 10 test.txt
4 5 24 total
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Expected:
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$ echo "some text" > test.txt
$ printf %s "one\ntwo\nthree" | wc - test.txt
3 3 14 -
1 2 10 test.txt
4 5 24 total
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