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In the received RAW codes, the width of the modulated pulses is less than the expected value. And the width of the spaces is bigger than expected.

Cited from the documentation:

Depending on the type of IR receiver hardware you are using, the length of a mark pulse is over reported and the length of a space is underreported.

Based on tests performed by Ken Shirriff who wrote the original IRremote library upon which this library is based, the length of a received mark is about 100µs too long and space is 100µs too short. IRLib1.x used that value however my own experience is that 50µs is a better value so that is the default for IRLib2.

You can change that value based on your own experiences by changing the value for example…

myReceiver.markExcess= 75;

Please find this value experimentally (try at first 100). To do this, change the receiver's sketch:

void setup() {
  gotOne=false; gotNew=false;
  delay(2000);while(!Serial);//delay for Leonardo
  Serial.println(F("Send a code from your remote and we will record it."));
  Serial.println(F("Type any character and press enter. We will send the recorded code."));
  Serial.println(F("Type 'r' special repeat sequence."));
  myReceiver.enableIRIn(); // Start the receiver
  myReceiver.markExcess= 100; // by default is 50

And please increase the delay time in transmitter sketch (to avoid codes overlapping) :

for (int i = 0; i < 3; i++){ 
  irsend.sendNEC(0xFF6897, 32); 
  j = j + 1;