The escape sequence for a tab doesn't seem to get along with a colon very well.
Code:
v:\> set a=1
v:\> set b=2
v:\> echo %a^t%b
1 2
v:\> echo %a:^t%b
1:t2
v:\> echo %a: ^t%b
1: t2
v:\> echo %a: ^t %b
1: t 2