u/Background-Towel-760

My 5 year old son has a vision issue picked up at a school screening and got a hospital appointment booked for him. However I took him to the optitions 4 weeks beforehand. In this appointment they found a prescription of moderate long-sightedness and a -1:00 astigmatism in the left eye but none in the right. However at the hospital screening, they found the same level of long-sightedness but an astigmatism of +1:50 in the left eye but +1:75 in the right, where there was none found before! What could be the reason for such difference in prescriptions only 4 weeks apart?