If you're using EasyStep Mode, then on the Registered Savings Plan Profile check off “Have unused RRSP/PRPP contributions or unused Specified Pension Plan contributions from prior years”. Then as you continue through the pages, you’ll get the option to view and change your unused contribution amounts.
If you’re using Forms Mode, you can see & change the amount on the Carry FWD form. Look near the bottom of the 1st page.
As to why it happened – if the amount was incorrect on your 2023 return, then when you transferred your information to 2024, the erroneous number would have been entered into your 2024 return.
