A typo like "cuberoot" for "cube root" should not be ignored.

A space was introduced in a file, replacing the one word "cuberoot" by two
words "cube root". With "Ignore whitespace" set, the change was not shown
in SourceTree. It was, of course, visible when "Show whitespace" was
selected.

It looks to me as though SourceTree is using the git diff option -w:

-w, --ignore-all-space
Ignore whitespace when comparing lines. This ignores differences
even if one
line has whitespace where the other line has none.

rather than -b:

-b, --ignore-space-change
Ignore changes in amount of whitespace. This ignores whitespace
at line end,
and considers all other sequences of one or more whitespace
characters to be
equivalent.