How come this rank up gem cost zero gold, while mine cost 407, 056 gold?

Igozorus

Screenshot from Seatin's video on Spider Gwen gameplay. You can see, with his Variant rank up 3 to 4 gem, it costs zero gold. How come? I had and still have a few 3 to 4 gems and they cost standard amount of gold as per usual rank up - 407,056 gold, just don't require resources. Can someone explain why Seatin's gem is zero gold?

Patchie93

The 2015 only r3-r4 gems from v1 and v2 cost 0 gold.
All other gems will be normal gold cost to rank up