To impute something is to assign that thing to a person or an object. So, for example, we could “impute the auto accident to a faulty carburetor” or we could “impute the auto accident to the driver’s failure to pay attention to traffic”. When used in a legal sense a responsibility is transferred from one person to another, as in “the theft was imputed to the child’s father”.
In theology the perfection of Jesus, His sacrifice and His “righteousness” is imputed to those who believe in Him. That is, His propitiation and atonement transfer His standing before the court of heaven to us as an imputed state.
All Christians have the same imputed righteousness. Not all Christians have the same manifest righteousness however. A Christian who yields to the indwelling spirit, who “crucifies the flesh”, who resists the devil, draws near to God, puts off the old man and puts on the new man is going to look and act very differently than a Christian who is luke warm and negligent.
All Christians may escape the final judgment, but they will not escape the judgment seat of Christ. Our fruitfulness on earth and our reward in heaven are determined by our manifest righteousness, not our imputed righteousness.
For example: 1 Corinthians chapter 3 has this teaching:
We are a team…a body with many parts…”one plants and another waters, but God gives the growth”. But (verse 8), “ He who plants and he who waters are equal, but each shall receive his wages according to his labor…(verse 13) “every man’s work will made manifest for the day will declare it because it shall be revealed by fire and the fire will try every man’s work of what sort it is”.