Need help working out this mistake

Your post is slightly confusing, as you do not indicate whether the bookie odds listed are those that you thought you took, or that they changed to.

However, I think what has happened is that the bookie odds have increased from 2.5 > 2.62 based off the numbers you have provided.

So, if I understand correctly, you have:

BACKED £25 @2.62 = £65.50 return (you were only expecting odds of 2.5 anda return of £62.50)

LAYED £24.61 @2.56 = £63.00 liability

If the bookie bet wins you will therefore be +£2.50 (£65.50 – £63.00)

If the bookie bet loses you will be -£0.88 (£25 – [£24.61*0.98])

Couple of other points if I may:
1 – this was not an arb until the bookie odds rose unexpectedly, which you seem to think it was

2 – even at the increased odds, it is a crap arb that is not worth bothering with unless this just happened to be a bet you needed to back and lay to qualify for something else

3 – you should make sure that the settings on ALL your bookie accounts are such that you do not automatically accept changes to the odds that occur between selecting something and clicking through to the bet slip to place the bet. Perhaps you do have that setting turned off for and reduction in odds (in this case it looks to have been an increase in odds) but it’s worth checking

4 – you should be able to work this kind of thing our yourself tbh – you were armed with all the facts and figures, the rest is just basic maths. That’s not meant to be patronising or to discourage you, but it is best to try and understand the mechanics and maths of betting and odds yourself

HTH 😉