But the manufacturer did not use the particular sample of the printer or the inks or the media you are using. All those things experience process variations from batch to batch. And printer heads age, etc.
On one hand, the manufacturer measures slightly non-representative targets not made on your exact printer with very high precision devices. On the other hand, end users measure exactly representative targets with lower precision devices. Both those scenarios have built in errors, but from different sources.
My experience has been that my profiles are a little better than the manufacturer's profiles for all of the various canvas media I use, particularly in lighter, low saturation colors and in their ability to exactly represent deep shadow tonality as seen on my calibrated monitor. And I know a few cases where profiles from highly regarded manufacturers could only have been mislabeled.