关于Layer层问题，为什么 Debug View Hierarchy 下不是整个填充视图？

Question

我自定义了一个BWLabel，主要实现画三条边框，重写了draw方法
- (void)drawRect:(CGRect)rect {

CGRect frame = self.frame;

UIBezierPath *linePath = [UIBezierPath bezierPath];

[linePath moveToPoint:CGPointMake(0, 0)];
[linePath addLineToPoint:CGPointMake(frame.size.width, 0)];
[linePath addLineToPoint:CGPointMake(frame.size.width, frame.size.height)];
[linePath addLineToPoint:CGPointMake(0, frame.size.height)];

CAShapeLayer *lineLayer = [CAShapeLayer layer];
lineLayer.backgroundColor = [[UIColor whiteColor] CGColor];
lineLayer.lineWidth= self.lineWidth;
lineLayer.strokeColor = [self.strokeColor CGColor];
lineLayer.path = linePath.CGPath;
lineLayer.fillColor = [self.fillColor CGColor];

[self.layer addSublayer:lineLayer];

}

出于好奇，我使用 Debug View Hierarchy 去查看视图层次，发现，在 Debug View Hierarchy 上layer并不是整个填充Label。我之后又尝试用UILabel，设置背景色，也发现同样的问题，我很好奇，这是为什么？