It means there is some packet loss, but that's ok, if it's a 'stable' 7hz that doesn't seem to vary much it could be busy channels at your location that aren't getting through, You could check by doing a pair of RF Nanos with a different 16 channels to your usual ones. It could be something resonant at a particular frequency that's 'absorbing' a channel. Also, is the measurement method accurate? Essentially servos need 50 updates per second and your set is receiving 140 so plenty of redundancy left!

Cheers
Phil