I have the following array called $fruits:
Array
(
[response] => Array
(
[errormessage] => banana
)
It just boils down to PHP's crazy type system.
$fruits['response']['errormessage'] is the string 'banana', so you're attempting to access a character in that string by the ['orange'] index.
The string 'orange' is converted to an integer for the purposes of indexing, so it becomes 0, as in $fruits['response']['errormessage'][0]. The 0th index of a string is the first character of the string, so for non-empty strings it's essentially set. Thus isset() returns true.
I don't know what you're trying to do in the first place so I can't offer any "fix" for this. It's by design.