If by “shark-fin patterns” you mean the characteristic sharp rise/slow fall (or the reverse) seen in VLF SID recordings during a solar flare, the reason some observers see the fin pointing up and others down is usually the geometry of the VLF propagation path, rather than one observer detecting a fundamentally different solar event.
A solar flare increases X-ray/EUV radiation reaching the dayside ionosphere. This increases ionisation in the D-region, changing the effective height and conductivity of the Earth–ionosphere waveguide through which the VLF transmitter signal propagates.
The important point is that the received VLF signal is a combination of several propagation modes. When the D-region changes, their amplitudes and phases change. At one receiving location those modes may interfere more constructively, so the received signal rises:
normal level → 📈 flare → gradual recovery
At another location, the same ionospheric disturbance can make the modes interfere more destructively, producing:
normal level → 📉 flare → gradual recovery
So an upward SID doesn’t necessarily mean “more ionisation gives a stronger signal”, nor does a downward SID mean less ionisation. Both can indicate increased D-region ionisation.
This depends particularly on the transmitter–receiver path length, transmitter frequency, location of the path relative to the Sun, time of day, season, and modal interference pattern. Two observers receiving the same transmitter can therefore record the same flare with opposite-going shark fins.
There is an additional interesting consequence: even at your own observatory, a particular transmitter need not always respond in exactly the same direction. As the propagation geometry changes through the day and year, a flare response can become small, change shape, or potentially reverse sign.
For SID work, therefore, the scientifically useful quantity is often the departure from the normal undisturbed signal, rather than simply whether the trace goes upwards or downwards.