Model year vs registration year
The model year is set by the manufacturer and can start before the calendar year. A car built in late 2019 might be a 2020 model year, and vice versa.
The registration year is when the car was first put on the road. The two are usually close but not always identical, which is why a listing's year and the VIN's model year can differ without any fraud.
Reading the 10th character
Position 10 of the VIN encodes the model year using a letter or digit. The letters I, O, Q, U and Z are skipped to avoid confusion.
Important: on many European BMWs the model year is not reliably encoded at position 10, so a decoder may return it as undetermined. That does not mean the VIN is wrong; it is a known limitation for some markets.
Why it affects the deal
Model year influences residual value, equipment level and sometimes warranty eligibility. Knowing it before you negotiate means you are comparing like with like and not overpaying for a year the car is not.