Doesn't sound right to me.
It doesn't matter if the amount is a reduction in A or an increase in B, its the same amount being evaluated.
You will get compound interest savings on 1 and compound interest on the other.
Can't validate as your numbers could be all over but £233-£337 is an average of £285. So thats depositing (or overpaying) around £6800 over two years.
As its a steady stream and only 2 years a simple check of £6800 x 5.17% /2 is £175 interest at 5.17%. It will be a little higher with compounding but not £721.
So neither figure on a quick check sounds likely to be correct to me
But
maybe the £248 is if the repayments skew towards the top end of your additional range a lot more than the bottom.
For perspective, just added a little extra, the first months £285 will earn £30 interest over 2 years. The same £285 will save £24 mortgage interest. Which includes compounding (but is rounded for simplicity).