Probably a very old installation, and grandfathered into the modern regulations, as there pretty is nothing about having to upgrade all installations electrically as the regulations change, only that it must comply to the regulations applicable when built, and that anything that is done repair wise must be like for like, otherwise it is an "upgrade" and the upgrade must comply with current regulations.
Thus here you find a house that is a century old, and which has been in the same family for that time, and thus still has the original fuse box with wire fuses in it, fuse wire on a card for the replacement of them, and gutta percha wire though the house. Still complies, till you want to change a plug outlet, because 5A socket outlets are pretty rare, nothing easy to find plugs for, and the bakelite has disintegrated. Then you have to pull in new cabling to the distribution board (and find the @#$%^ sparkie cut cost, because he only pulled in a new red live wire instead of pulling in new line, neutral and an earth conductor), put in a new distribution board with mains isolator, earth leakage and 20A breaker, fed from the old board as a sub board, to get the installation to comply with the modern regulations. Then you find the bonding on the water pipes has failed, the grounding at the meter has rotted away, and the only ground the system has is the rather dodgy wire on the overhead 3 wire feeder that leads to the light pole outside, original to the house 90 years ago, and , as it falls on the boundary, is not checked by home owners, and ignored by the Metro electricians unless the wires have all fallen off.
Biggest shock on selling the house is the electrical compliance, as often you then have to replace everything to get it up to code, as the act of checking the wiring inside the sockets, switches and lights is often enough to cause the brittle insulation to turn to powder.