Chroma is stored at half the resolution of luma in consumer formats (called 4:2:0 sampling, found on DVD, blurry, streaming, etc.) so you first need to scale the chroma to match the luma before the conversion to RGB. You often want to use a different method for this compared to scaling both the chroma and luma together, as is done by image upscaling.
Referring to the image in the third post might help you understand the steps needed.
