1970/1/2 Vin and model number question

[72_longbed_Stepside]

Hey everyone, I am a newer member here, and this is my first post. My dad and I at the end of last year bought a 1972 chevy Long bed step-side with the original 307 it has been converted to a 3 speed floor shift. Now today a matter of a few miles form our house a 197? k10 short bed step-side with a 292 and 3 speed column shift came up for sale. So now we own 2 classic Chevys! Anyway I took a picture of the build sheet(?) on the inside of the glove box. SO now my question, looking and comparing the vin and model number on the sheet the vin says a 1971 but the model number says 1970? I am almost sure I should go with the vin but I just want to know why they differ? Thank you in advance!