Struggling with this one!

I contacted ID via the online chat - the only way ,  I ordered a payg SIM card in April, but didn’t use it until June, by this time the credit had gone. The response I got from ID was 

Zintle: Kindly be advised that in June you didn't purchase data you purchase a top up and you got a text that says "Just to let you know that you now have £1.78 credit remaining on your account. Don't forget to use the iD Mobile app or My Account online to keep on top of your usage and bundles."

what is the difference in a top up and purchasing data? I’m totallY foxed!