Hi. Welcome.
I think that’s because your staking size is so small it doesn’t have room to do it in any other way. This would be real arb if your stakes were 17.72 and 22.28 – but that would look suspicious for the bookers, because normally people won’t use such stakes – so the software is not suggesting it for you. It has a rounding feature.
(17,72 x 2,30) = 40,756
(22,28 x 1,83) = 40,77
See, with stakes 17,72 and 22,28 you would’ve made guaranteed profit of either 0,756 or 0,77 £.
However, because your whole stake is only £40, it can’t round the stakes in any better way so the only option is 20-20, which is causing this issue. If your stake per game would be 200 for example, it would be easier to balance it accordingly between the bets (for example 90-110).
(90 x 2,30) = 207
(110 x 1,83) = 201,30
Still not the same outcome, but at least you’re winning on both.
40 is very small anyway. It’s a 1,91% arb so much bigger stakes would be recommended to make it worth your time. Are you really willing to place two bets for 0,77 £ profit, even if it’s guaranteed?
However, what would be even more recommendable in my opinion, would be to start exploring value betting side which is more lucrative.