The guts of it is: cold day and high pressure - you're getting a lot
more oxygen into the cylinders - vs a hot day and low pressue.

While the QNH along the route is variable, it generally doesn't vary
*much* - ie you get to another 'area' and it's an hectopascal or two up
or down. Presumably you were flying with a high QNH (well above STP
value which is 1013 hectopascals at 0' AMSL (1013.2 to be precise,
IIRC)).

--
Duncan

On the other hand, cold high pressure is usually descending air, so the
aircraft is sort of swimming upstream just to stay level.