how to escape inside reg.exe windows registry - strange behavior

Question

Everybody say that \ works as an escape inside reg.exe like:

The REG command will interpret \ as an escape for the character that immediately follows it. source

but I found it not a full correct rule. see this examples please:

escaping a character does not work:

reg add HKCU\testt /f /v "" /t REG_EXPAND_SZ /d "a\b"
===> a\b

escaping the escape does not work:

reg add HKCU\testt /f /v "" /t REG_EXPAND_SZ /d "a\\b"
===> a\\b
reg add HKCU\testt /f /v "" /t REG_EXPAND_SZ /d "a\\b"
===> a\\b

but when I escape a double quote " then escaping the escape will work:

reg add HKCU\testt /f /v "" /t REG_EXPAND_SZ /d "a\\\"b"
===> a\"b

conclusion: \ works as an escape only if I escape a double quote "

and more strange is this, escape the escape does not work even when I escape a double quote:

reg add HKEY_CURRENT_USER\testt /f /v "" /t REG_EXPAND_SZ /d "a\\ \"b"
===> a\\ "b

this means that to escape the escape it have to be adjacent to the escaped double quote!!

is this a correct behavior? where is this documented if it is correct?

the examples were done on win7 using cmd.