That pot/soil might have an abnormal level of salts; most "moisture meters" are essentially Ohm-meters registering electrical resistance (or conductivity - the inverse, so same thing by another name, really) between the electrodes.
Salts would greatly increase the conductivity/reduce the resistance .vs. a less-salty mixture. Alternatively (perhaps the soil is all the same between pots?), the pot itself could be abnormally conductive...
But it might just be that the original plant (and core soil) in this pot had been somehow rendered salty. Now, if this is the only thing you are fertilizing with "citrus fertilizer" it's possible that acids (or some other component of the fertilizer) rather than salts are what's impacting your conductivity in this pot.
I personally find most of the various electronic meters that are available for gardening to be dubious, at best. And it's a bit expensive to equip your pots with a proper tensiometer that actually measures a "direct" moisture readng, rather than inferring from some electrical parameter than can vary for other reasons.