IPhone : NSNumberFormatter glitch

I have a strange problem. It only seems to present on a tester's iPhone 5s. It works correctly on an iPhone 5, 6 and 6 plus, all running the latest iOS (8.3).

This is the code

-(NSString *) commaString:(double)number
{
    NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
    [numberFormatter setNumberStyle: NSNumberFormatterDecimalStyle];
    [numberFormatter setGroupingSize:3];
    [numberFormatter setMaximumSignificantDigits:9];
    NSString *numberAsString = [numberFormatter stringFromNumber:[NSNumber numberWithDouble: number]];

    return numberAsString;
}

The application is a calculator and is usually presenting correctly like this

It is usually showing correctly like this

but on this iPhone 5s it is showing like this

I thought the setMaximumSignificantDigits would have nipped this in the bud, but it's still showing the same. Could this be some strange localisation thing? I don't think it is as his iPhone 6 it is showing correctly also.

Thanks

Luke