Liam has the higher average because his median is to the right of Vicki's median. The median is visually shown as the vertical line inside the box. For Liam, his median is 15. Vicki's median is some value between 10 and 15 (closer to 10).
Liam is also more consistent. How can we tell? Note how Liam's box and whisker plot has a shorter/smaller box. Also the min and max are closer together as well. The range is smaller for Liam. The interquartile range (IQR) is also smaller for Liam. Liam's scores are closer together. Vicki's scores are more spread out due to a larger range and IQR.
Notes
1) The IQR is defined as the horizontal distance from the left edge to the box to the right edge
2) IQR = Q3 - Q1 where Q1 and Q3 are the first and third quartiles respectively
3) Q1 is the left edge of the box; Q3 is the right edge of the box