为什么sizeWithFont：和sizeWithAttributes：返回不同的值？

Question

我有CCLabelTTF和NSString文本。在5日IOS sizeWithFont：返回高度大于sizeWithAttributes：7日。

if ([[[UIDevice currentDevice] systemVersion] compare:@"7.0" options:NSNumericSearch] == NSOrderedAscending)
{
     dim = [string sizeWithFont:font];
}
else
{
     dim = [string sizeWithAttributes:[NSDictionary dictionaryWithObject:[UIFont fontWithName:name size:(CGFloat)size] forKey:NSFontAttributeName]];
}

dim differs on 1 pixel for 5 and 7 ios