Measuring floodlights at school with "THALES"

Measurements of the first group:

• Shadow of the floodlight 2.97 m
• Shadow of the student 0.86 m
• Height of the student 1.60 m

Two groups of students of 3º A have been measuring a floodlight using Thales Theorem. They used a tape meter and mesured the shadow of the floodlight and the shadow of a student, they mesured the height of the student too.

Measurements of the second group:

• Shadow of the floodlight  3.17m
• Shadow of the student 0.84 m
• Height of the student 1.60 m
  

So there is a diference of 51cm, Why do you think they have obtained different values of the measurement?