Why does a token starting with '=' change semantics of a command in zsh?

The following shell command requires no explanation

a=1

However, when an extra space is added, zsh gives an unexpected error

a =1
zsh: 1 not found

Could someone explain this, please? What is the meaning of ‘=’ is zsh?

Bash produces an error that I expected:

a =1
bash: a: command not found

>Solution :

The fine manual says:

`=' expansion
   If a word begins with an unquoted `=' and the EQUALS option 
   is set, the remainder of the word is taken as the  name  of  
   a command.  If a command exists by that name, the word is 
   replaced by the full pathname of the command.

Thus:

%echo =ls
/bin/ls