[time-nuts] Is this amount of measurement errors to be excepted when measuring with a small frequency difference?

[Erik Kaashoek]

During testing of some oscillators that where not exactly on the right 
frequency it appears that there is a measurement error in the frequency 
measurements every halve the inverse of the frequency difference between 
reference and measured signal when the gate time of the frequency meter 
is set to 20ms.
An example plot can be found here: 
http://athome.kaashoek.com/time-nuts/Frequency_pulling.png
It shows the measured frequency with 0.5Hz (blue) and 1Hz (pink) 
frequency difference.
The normal frequency variations measured with a gate time of 20 ms are 
below 2e-9 but when (presumably) an edge of the reference and the input 
signal coincide(?) the frequency deviation can go up to almost 1e-8.
As you can understand this effect is not visible with a larger gate 
time, such as above 50ms
Is this normal behavior for a frequency meter?
Does this imply that when you measure a signal that is phase locked to a 
reference with a 20ms gate time you may have substantial larger noise in 
the measurement because you have locked the system in the worst case 
phase relation?
The frequency meter used is a Picotest U6200A with an external 
reference. The results are the same when using the internal reference 
but the internal reference has much more phase noise obscuring the 
effect a bit.
Erik.