Changing Entry to Standard subscription

Recently, I changed my recurring sub from Entry 30 days to Standard 180 days. It was already 4 days past since a billing cycle. I'm paying with credit card.

So SE charged me £1.12.
I contacted SE support, they informed me that I'm charged pro rata and some extra more for the upgrade to Standard and rate based on Standard 90 days rate of £8.39 per month. Both Entry 30 days and Standard 180 days are £7.69/month.

This don't add up.
I highly suspected £1.12 came from the 4 days past: 4 days ÷ 30 days × £8.39 = £1.11867
If I were charged pro rata for the remaining 26 days as Standard sub, it would be: 26 days ÷ 30 days × (£8.39 - £7.69) = £0.60667

Now I understand it may be a small amount, but I don't freely place my trust on people who don't know their maths and attempts giving lame excuses.

Anybody who faced this issue too or is it just me SE try to screw me out of a meal?

Overall the game is great but I reverted to Entry sub and will remain so.