For a constant engine temperature, you will get more and more thrust as ambient temperature reduces.
On a cold day, engines generally produce a whole lot more thrust than the (commercial) airframer really needs.
There at least two good reasons why the airframer might want to limit his max thrust by "flat rating":
1. Minimise minimum control speeds (Vmc), i.e. help the rudder in its fight against asymmetric thrust in an engine-out case, by defining an artificial max "flat rated" thrust that is not exceeded.
2. Minimise structural loads on the pylon/thrust attachments, i.e. why have massive thrust on a cold day, which you'll probably use very rarely, when you're going to need big, heavy, over-engineered lumps of metal to make sure the attachment is strong enough?
I'm sure there are other reasons, but generally why burn up the engine on a cold day when the extra thrust gives you more problems than benefits.
The temperature below which thrust is flat rated varies significantly from aircraft to aircraft, but is generally between ISA and ISA+20C.