That may be so over the last 30 to 40 years Alvin but if you look at the last 10 years , both seem to be making up for lost time ..
December is a bit of an odd one as we had many very mild ones in the 1970s and 80s and then for some reason a complete dearth in the 1990s and 2000s which also brought many sub 4C examples . And of course we had 3 cold ones from 2008 to the legendary 2010.
Since 2010 though Dec has been very mild indeed .
Meanwhile we see the opposite for the other end of winter ie Feb and March had alot of amazingly warm editions in the 1990s and early 2000s but have calmed down somewhat since and managed some colder editions too .