C array assign & access - c

Question

I'm writing C code in a PIC16F1824, and are using UART.
I had this for receiving:
for(int i = 0; i < 9; i++){
while(!PIR1bits.RCIF);
RX_arr[i] = RCREG;
while(PIR1bits.RCIF);
}
RX_arr is an array declared as int RX_arr[9];
RCREG is the UART receving register and is supposed to have
0xFF 86 00 00 00 00 00 00 47;
Question1:
When i = 1, RX_arr[0]'s value(0xFF in this case) also changes when RX_arr[1] = RCREG is executed(Both postion 0 and 1 have 0x86).
Later I found using this segment of code instead of the for loop above will successfully assign one byte into each position:
if(!RCSTAbits.FERR && !RCSTAbits.OERR){
RX_arr[0] = RCREG;
}
However, the byte saved is wrong, only the 0xFF is always correct, other bytes are some other values. I found the OERR bit is set, which indicates an overrun error(2 byte FIFO buffer is full before accessed). How can I receive all the bytes in RCREG?
Question2:
After I get the response in question1, I read the useful information in it, which is in byte 2 and 3, and I made a variable called data. In my case the data is a sampled temperature, and I want to collect 20 samples,and save them in an array called ppm_array. sum_ppm is supposed to be the sum of all 20 samples, and will add the new sampled temperature to it every time a new sample is collected. When I run sum_ppm += ppm_array[i](sum_ppm = 0, ppm_array[0] = 1616, and the rest of the positions are 0),I got sum_ppm = 4508160. Why isn't it 1616??? Am I adding the address of them?
data = RX_arr[2] * 256 + RX_arr[3];
ppm_array[i] = data;
sum_ppm += ppm_array[i]
Many thanks.