The issue is your mains ground may be rubbish.
Mains grounding starts at the Ufer-in-foundation, grounding rods, or water pipe connection (Oh, the things that can go wrong with water pipe connections!!)
From there, it goes to your Grounding Electrode System, the above plus the wiring which carries the grounding to the main service panel.
In the main panel is the Neutral-Ground equipotential bond, which clamps neutral near earth and assures the transformer secondary doesn't float at high voltages due to capacitive coupling or leakage. It also gives fault current on ground a way back to neutral. This thing breaking can cause problems.
From the main panel, safety ground is carried onward to any subpanels and to the individual branch circuits. Then, up each branch circuit one by one, each connection daisychaining off the next.
Where can this go wrong? Oh, so many places.
- The grounding source may have an issue because the water company installed a plastic smart meter.
- The grounding electrode cables may be disconnected or corroded.
- The ground wire from the panel may be broken.
- The ground wire for a subpanel circuit may have been landed on a neutral bar, because that's allowed in the main panel (since it's just a funny way to do the equipotential bond).
- The ground wire may be broken.
A group of receptacles may have islanded grounds, when a 2-wire circuit is extended with 3-wire (w/ground) cable; in the addition area, people typically connect grounds to each other, but don't bring the grounds back to the main panel. In this island of grounds, any ground fault lights up all the grounds in the island, because it can't carry current back to the main panel and the N-G equipotential bond.
The ground may not even be present at that receptacle. Pre-1960s wiring didn't have ground wires. Sometimes people just slap a 3-prong outlet to replace a 2-prong, so they can plug in their equipment, and leave the ground to dangle. It's the "islanded grounds" problem but just within that receptacle.
Ditto, but protected by a GFCI, which is a legal way to leave 3-prong outlets ungrounded. This requires a sticker stating "no equipment ground", but people tear off the sticker because it's ugly. Anyway, this "ground" is useless, but at least the circuit is less likely to kill you.
Bootlegged ground. Sometimes for lack of ground, people connect ground to neutral so that 3-lamp testers will "pass" the receptacle. This is nasty, because if the neutral wire breaks, the ground is energized with mains voltage.
Someone installing a workshop subpanel off an old NEMA 10 (hot-hot-neutral) circuit that went to a dryer or range. No grounds here, so the whole subpanel's grounds are either islanded or bootlegged!
But yeah, if your grounding is tip-top, then grounding your ESD mat/strap is the right thing provided you do it through a 1-megaohm resistor. The point of that is so if the worst happens and either the ground is energized or you touch mains, current flow is impeded below dangerous levels.