Why do you have to charge for multiple exposures of X-ray films when taking X-rays?

The charge for taking X-rays is as follows: for example, a film is exposed once in 50 yuan and twice in the same film, such as taking a lateral position of limbs. The charge is the first exposure fee of 50 yuan, plus the second exposure fee, such as 10 yuan, then 60 yuan will be charged. This charging method is approved by the local price bureau, and there will be no problem in regular hospitals.