Robert Woolley wrote:
Easy:

a) Lower usage - on average you've got one train every 10 minutes on
Great Northern Electric Monday - Friday until 20.00. No service Sats
and Suns.

This means less heat from trains and passengers

b) Length of tunnels. Highbury is not too far from Drayton Park and
Essex Road - so air is getting pushed in at this point. The Victoria
Line has ventilation shafts, but trains only join the line just north
of Seven Sisters.

I suspect it is more complicated than that, the WAGN platforms are very
damp, and the 2 are of diffrent construction