.usesFontLeading gives a different height when used with boundingRect

I'm using boundingRect to calculate the height of a String, I'm using Ubuntu as my font, and when I supply .usesFontLeading as one of the options alongside .usesLineFragmentOrigin the height is completely different to what I expect.

Here is the code I'm using for reference:

func height(withConstrainedWidth width: CGFloat, font: UIFont, maxLines: CGFloat = 0) -> CGFloat {

        let constraintRect = CGSize(width: width, height: .greatestFiniteMagnitude)

        let boundingBox = self.boundingRect(with: constraintRect, options: [.usesFontLeading, .usesLineFragmentOrigin], attributes: [

            NSAttributedString.Key.font: font

        ], context: nil)

        var size = boundingBox.size

        if maxLines > 0 {

            size.height = min(size.height, CGFloat(maxLines) * font.lineHeight)

        }

        return ceil(size.height)

    }

Firstly, what does .usesFontLeading even do, and why is it that the result I'm getting an unexpected result.

Note: self is String because I've extended the String class.