I think that the DHT11 should not be connect on +5V in a permanent way. I had it connected on 5V in the past and had humidity data off (low) by about 10%.
Now I use one of the digital output of the Arduino to supply it with 5V for 1 or 2 seconds every 20 seconds. Results are now more constants and higher by 10% (for 2 units)!
Note: the resistor between pin 1 and 2 is also connect to the output.