*GENERAL REPLY*
From my observations of how mIRC treats pasted text, I infer that mIRC doesn't count a line of text as a line until a carriage return occurs. I.E: You press the enter key, or the copied text contains a carriage return.
Therefore, by this inference, a line as simply 'A' is not a line until the enter key is depressed, or a carriage return is implemented in a copy+paste method.
I hope that makes sense.
Following the above inferences, it would be obvious that this is what the line numbers are:
'A' - Line 0
'A [carriage return]' - Line 1
If you really missed what I'm saying, its this:
A line of text is not a line until the enter key is depressed, or a carriage return exists at some point in a copy+paste event.
However, this is pure observation.