I see what happened. They don't count the number of days you were living together. On 07/21/2018 (beginning of statutory period) when the 3 years preceding the filing of the application you should have been living together. Just on that day but you weren't hence the denial. They only covered period 07/21/2018 - 07/21/2021. Had you applied on 08/23/2021 you would have been fine because your statutory period would have started on 08/23/2018.