Is informing a person that he scored 90% on a certificate, when his actual score is 89.73%, considered a lie, knowing that there is a benefit for him in that?

If the intention behind saying "I got a full ninety" was to register in a college that requires a ninety, then what you obtained is given the ruling of ninety based on the principle "what approximates a thing is given its ruling." In this case, we hope that there is no sin upon you, noting that it would have been better to use allusions and circumlocution.