I believe the meter has been installed correctly. How could it have been installed incorrectly? It looks like an error on my behalf but im not 100% sure.

Hi All, I recently noticed that my economy 7 meter readings are the wrong way round e.g. my night time reading is the day and vice versa. If corrected this means i will owe around £1000! I have been entering my own reading online for 3-4 years now and believe i must have somehow switched the reading when they were both similar although im not sure. Is this solely my fault? someone comes round every 6 months or so and checks my reading shouldnt they have noticed? Thanks in advance