The sensors are very big. The sensor is way bigger than that little red square. If you are using the centre focus point, the focus sensor stretches out too the () markings of your spot metering markings. You can use this as your focus point, rather than the red square. Sinnce the shrubs hold more contrast than a bland face it has focussed on the background rather than the girl. The girls face is very small in the picture so I am sure the sensor would have over lapped. It will focus on whatever or wherever it sees the most contrast. You should have focussed on her stripey shirt. This is a bigger target and high on contrast.