The electricity supplier is only concerned that the voltage is uniform up to the front door. Once you open the door and let it in, the internal house wiring, and/or the wiring in your own "distrubution system" can introduce additional voltage drops.

All it takes is one under gage wire, or one bent, broken, abraded, twisted, pinched, or poked cable, and the voltage at the downwind end will change with every fluctuation in the current being drawn through the weak section - especially if your setup draws fairly high current, or if your rig needs more current than the house socket is meant to provide.

Usually, with good cables, it would take rather large variations in current to one load (your dimmable lights?) to affect voltage to your other components (your amps?), but cables are always suspect, since they get dragged around, coiled and twisted, stomped upon, etc. Connectors probably are the second most likely culprits, if the problem is to be found in your own kit, for the kind of variations described.

The venues themselves may have some over-large loads, with "industrial-strength" microwaves or large ice-machines being the usual suspects in my area. If you're in the UK where they use the trad Lucas Electric refrigeration units, I'm told you can discount the ice-maker 'cause they never work anyway, but someone might have patched one up. (?????) It would seem unlikely, though, that two different venues would have similar enough loads to look like "the same thing" in both places.

John