Calibration seeds your MMR — it isn't an open climb
Calibration is the run of 10 ranked placement matches that seed your starting MMR, either on a fresh account or when an old one becomes eligible to recalibrate. That's the difference from a normal climb: each placement game swings your rating far harder than the ±25 or so you gain per match afterwards, so the 10 games together set a starting medal in one batch instead of grinding a rating up point by point. Because the placements are front-loaded leverage, the whole point of boosting them is to convert that leverage into the highest honest seed your account will support — after which the climb (if any) is a separate, gentler job.