ok, in that case, what's happened is that you have estimated x amount for you, y amount for your other half - your combined income is above last year, but one of you is under and the system is suggesting increasing one of those estimates


as an example (using very rounded numbers!)


estimated 10k for you, 60k for partner - 70k in total
actual last year was 30k for you 40k for partner - 70k in total
BUT, the estimate for you is waaaaaaaaaaaaaaay under the actual
SO
system sends out letter saying it needs to be fixed, you've underestimated
suggests 32k for you (to allow for inflation), no change for partner (as his 60k is well above the 40k he earned last year)
gives you an estimate of 32k for you, 60k for him, total 92k

you need to make sure the estimates are ok for both of you individually, not just your total together kwim?