SSS - Some Signals Slope

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VE9DAN
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Last seen: 4 weeks 1 day ago
Joined: 2011/11/12 - 00:38
SSS - Some Signals Slope

I notice on the WSPR 'Waterfall' that most signals in the 3 minute slot run absolutely horizontal.
But then, some signals seem to slope up or down slightly.
Why?

Dan, VE9DAN