I don’t understand why an APC was paid for one of my articles but not for the next one.

Funding decisions are based on the specific journal and funder policies which are often unique to each paper. If you publish in a different journal or acknowledge different funders, it might change what the best option is.

The conditions on our block grants are also quite complex and do change over time, so authors should not assume that any open access fee will be paid by us. The current University of Cambridge funding guidelines can be read here.