Transaction Fees don't seem to work out

[QRI]

Hello, I cant seem to make any of the transaction fees fit what we are being charged at the minute.

2.90% + 30p doesn't work, neither does 1.5% + 10p or 2% + 5p

Here are 3 examples below

£41.46 paid - 0.78 fee = £40.68

£26.17 paid - 0.57 fee = £25.60 (2% + 0.05p works for this one) - coincidence?

£31.00 paid - 0.63 fee = £30.33

Anybody have any ideas?