Instruments false positives

I have been able to get rid of reported memory leaks in Instruments not by fixing a leak but by adding some lines (like releasing a previous nil object) that convinced Instruments it is not a leak.

But I am stuck are some I cannot stop Instruments from reporting. It reports a leak in this code

if(expr.length>0)
{	// add previous Atomic
	Atomic *atom = [[Atomic alloc] initWithString:[string substringWithRange:expr] isNumber:aNum];
	[exprTokens addObject:atom];
	[atom release];
}

}

and it underlines the substringWithRange method. I changed code to explicitly release atom and I assumed substringWithRange would return an autoreleased new string. I am aware Instruments tells you where the leaked object is created and not where it is leak, but only things created here are atom and a sub string and atom is released immediately.

The initWithString method might do it, but I don't find anything there. Also, while running instruments, this code is 8542 times (I am pretty sure always in the same context) and Instruments says it only leaks 61 times.

I don't now if there is misunderstanding are a code problem?