So you say the meter indicates 20 kWh at night and this is about 75% of your daily usage? So it says something like 7 units in the daytime?
27 units a day for 365 days is about 10000 units a year, which is probably correct for an all electric household.
Like a PP says, check that the clock on the meter is correct, has am and pm been mixed up for example?
I wouldn't just think 'bonus, we're getting our daytime electricity at the night rate' because it could be that they'll work out later that your actual usage was different to what the meter is showing and come after you for the difference.
Years ago, we had the same problem, it was either that the meter clock was wrong, or that the meter was incorrectly wired, so it was showing the daytime use on the night meter and the night use on the day meter. I can't remember which, whether or not we built up a debt that they chased us for or how it was sorted out, but it means that this is something that can happen.
Check the clock on your meter and if it is wrong, there's your answer. If it is right, or there isn't a clock you can see, spend a few days recording the readings on both meters first thing in the morning and last thing at night to see if it looks like its the wrong way round. Either way, talk to your supplier to get them to sort it out.