It does make sense: that is the way how characters encoded in ASCII table: 0 char maps to decimal 48, 1 maps to 49 and so on. So basically when you convert char to int, all you need to do is to just subtract ‘0’:
scala> '1'.toInt
// res1: Int = 49
scala> '0'.toInt
// res2: Int = 48
scala> '1'.toInt - 48
// res3: Int = 1
scala> '1' - '0'
// res4: Int = 1
Or just use x.asDigit, as @Reimer said
scala> '1'.asDigit
// res5: Int = 1