Economy 7 - hours are slightly different to those specified

My Economy 7 seems to work slightly out of the hours described.
Having checked the changeover times on my meter, it switches over about 20 minutes later than the Economy 7 tariff.
Do I go by the hours specified, or is it OK to go by the switchover times on my meter?
Only asking because I set my dishwasher & washing machine to operate during Economy 7 times.
Thank you & Kind regards.

This results from jitter than is added or subtracted from the “official” time, in order to prevent surges on the distribution network when everything switches on or off at once. Consider when a large number of properties have storage heaters, if they all switched on at exactly the same time then where would be a huge surge load on the supply. To avoid this, meters are offset a few minutes either way to spread out the switching time.

You need to go by only the switching time on the meter. That’s the device measuring your specific usage and is the only authority on when your rates change.

Thank you, that’s really helpful and makes sense.