Unexpected results from Encoder::GetRate()

[LyraS]

I'm trying to set up a speed controller using the US Digital E4P-250-250-D-D-D-B encoder and the WPI Encoder class. Everything basically works fine but I'm seeing an odd discrepency between the values out of GetRate() and GetDistance(). Spinning the encoder at a constant rate, I would expect that the rate returned by GetRate() (units/sec) would more or less match the one-second delta returned by GetDistance(). But what I see is this: the distance returned for one second is almost exactly half of GetRate(). Because GetRate() is fairly noisy, I'm testing using a 20 sample moving average (with a sampling rate of 40hz). I'm sure I'm missing something obvious... Can one of the WPIlib experts here clue me in on what it is? Thanks!

Update, possible clue:

Looking through the WPI source I noticed this comment in Counter.cpp (used by 1x and 2x Encoder):

/*
* Get the Period of the most recent count.
* Returns the time interval of the most recent count. This can be used for velocity calculations
* to determine shaft speed.
* @returns The period of the last two pulses in units of seconds.
*/
double Counter::GetPeriod()