There are a number of reasons for the apparent change in recieved signal ,they include station keeping ( hydrozene fuel is limited and costly on board each satellite) of the satellite a very small positional change in the satellites position has a very large effect when you are at, or outside of the specified footprint. Also the actual science of the designed footprint from a satellite transpônder is not always exactly that with the desired design.
Addionally at any time the actual output power per transponder will vary equally over time as will also the satellites solar cell effeciency and its battery effeciency.
THE travelling tube amplifier (TWA) output as a consequence is also effected during the above cycles- one of the resulting differances also seen during times of reception.
All satellites are subject the basic conditions above, best option is always to conduct a complete survey at the proposed reception site.
PS i hope the link you identify is free for the information at satcure?