ios masonry make.top和bottom各为7.5后变成7.6666奇怪现象

父视图高度随子视图label高度变化而变化的效果，但是label的top和bottom约束都是7.5

    _topView = [[UIView alloc] init];
    _topView.backgroundColor = [[UIColor grayColor] colorWithAlphaComponent:0.2];
    [self.view addSubview:_topView];
    [_topView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.mas_equalTo(30);
        make.right.mas_equalTo(-30);
        make.top.mas_equalTo(80);
    }];
        [_titleLabel mas_makeConstraints:^(MASConstraintMaker *make) {
        make.right.mas_equalTo(60);
        make.right.mas_equalTo(-20);
        make.top.mas_equalTo(7.5);
        make.bottom.mas_equalTo(-7.5);
    }];

用的plus手机，结果label的top显示是7.6666，而不是7.5，我猜是因为3倍屏的原因，但是也恶心了吧，凭什么系统改了呀....
大有家碰到过这个问题吗？

https://www.cnblogs.com/wangbinios/p/5716887.html