Wrong forum Vodafone topup. Unfair. Topup 30 get 10 free. Topup 40 then no free 10

Hi,
I went online to topup my ready to go Vodafone mobile this week.

There was an offer that if I topped up by 30 euros, I'd get 10 euros free.

I topped up by 40 euros and still only got 40 euros, not the 50 euros I'd expected.

I paid 10 euros extra for the same amount of credit.

This to me is unfair.

Any ideas what is going on?

Thanks,
Gearoid.