I think from their Knowledge and where it is they know it will be higher than the equipment located there.
In Cali the stations closest to the earthquake were not as accurate or at least that is what I remember but they didn't have a stable platform to measure. Imagine standing on a shaky ladder trying to measure the shaking, you are shaking yourself. If your shake is in concert with the quake waves it will look weaker, if you are shaking on your ladder against the quake it will seem stronger.
Also the instruments are drilled an set in the ground and if it's in sand it would react different then if it's in rock. There are just a lot of variables.
Like flying, you have a ground speed and airspeed which are never the same, you have to adjust for the mass you are traveling in, the wind direction, crab over the surface, it's not like a car that you have a wheel turning solid over ground.