Isn't a tab (char(9) ) whitespace in the ANSI/ISO SQL-92 specification (Section 8.2, , General rules #3)?
The same code above on MySQL 5.5 gives the same result

Tab isn't "space", it is a character.
The same applies to hard/non-breaking space (char(160) in "normal" collations)

The ANSI-92 standard states in 8.2. item 3.a


  If the length in characters of X is not equal to the length
                in characters of Y, then the shorter string is effectively
                replaced, for the purposes of comparison, with a copy of
                itself that has been extended to the length of the longer
                string by concatenation on the right of one or more pad char-
                acters, where the pad character is chosen based on CS. If
                CS has the NO PAD attribute, then the pad character is an
                implementation-dependent character different from any char-
                acter in the character set of X and Y that collates less
                than any string under CS. Otherwise, the pad character is a &lt;space&gt;.


CS is "collating sequence". I'll assume no collation uses tab or non-breaking spaces as padding characters. So space it is...

Note that LIKE is section 8.5 (I mention this because of your previous question)