Yes it certainly can ... there is atmospheric noise, noise created by street lights, noise from radio transmitters and domestic ...
Every location will be different and the noise level can vary significantly.
I have seen some locations where the swing is as low as 3dB and others up to 9dB and typically 5 to 7 dB is where most are.
Get RouterStats Lite and set it to plot your SNT every 30 seconds over a 30 hour period and see what that shows.
Also, get a 2wire2700 modem/router which if conditions are right will sync at 3dB and hold sync to around -0.5dB. Then timing your resync can give you a significant improvement..
If you can do a plot and post a link to an image - even with your current, I should be able to suggest when you your resync.
M H C
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