Because you should have hung up after 59mins and redialed.
Free to giffgaff
You need to top-up by top-up voucher, top-up by credit /debit card or buy a goodybag from your credit /debit card at least every 3 months to receive free giffgaff to giffgaff calls. If you don't, other UK rates will apply as above. The free giffgaff to giffgaff calls last for 60 minutes per call, so you need to hang up and re-dial if you want to keep talking for free.