Meter.one_minute_rate is wrong and/or badly documented

[cbouilla]

The following code:

rate = Meter()
for i in range(int(65/0.05)):
     rate.mark()
     time.sleep(0.05)
print(rate.one_minute_rate)
print(rate.mean_rate)

yields:

12.744700289590714
19.916839105797937

Why does the one-minute moving average differs from the average over the first minute?