AlphaBeta already explained.
-1 is what you get if you Serial.read() when there is nothing in the buffer. Serial.available() said something was ready, but if only one char is ready, then you should only attempt one Serial.read().
65 == 'A'
-1 == don't read from Serial.read() if nothing is Serial.available()
66 == 'B'
-1 == don't read from Serial.read() if nothing is Serial.available()
49 == '1'
and so on.