variables not working as expected

[spiralofhope <spiralofhope@xxxxxxxxx>]

This works in bash, dash, ash and sash, but not in zsh:

a="echo foo bar"
$a

-
in zsh, this works:

a="echo"
b="foo bar"
$a $b

-
in bash, dash, ash and sash, this works, but not in zsh:

a=ls
b="-a -l"
$a $b

-
I tried variations, such as:

a=ls
b=" -a -l"
$a$b

But I still canât get it working.  Is this a bug?  Or is there a
compatibility mode which I could use?