Activation fee and credit?

I was coming to the end of my 18 month contract and noticed there was a new deal offered in the VM mobile app. I then noticed that I could switch to Gig1 for £36.64/mth (increasing in April), so I decided to go for that. I've just received a bill that has a £35 activation fee and £11.21 credit. Could a member of the VM team please explain why I've been charged an activation fee and what the credit is for?

TIA.