OK, I opened up the code.
The 8-bit signed type which you are looking for is called int8_t, not char. You would more often see the unsigned version uint8_t but each of those 8/16/32/64 bit types has both signed and unsigned versions.
Which Arduino is this for? If it's for one with native USB serial such as a Micro or Due, then you're working with the USB buffers, which can contain 68 characters, unlike the hardware serial buffers that (usually) only hold 64 characters.
The USB system will package up the characters into packets, trying to optimise the performance of the link. That's how you get a large number seemingly appearing at once. It's actually working at a much higher baud rate than the 115200 that you asked for. That number is ignored when using native USB serial.