I take it you are referring to the hammer spring grease on a HW100 rather than the actual trigger unit?
The grease affects the hammer travel speed which reduces velocity in colder conditions, this should lead to a POI change rather than a change in the group size.
It's always a good idea to clean this off and run a dry lube like GT85.
If it's on the trigger unit then it should make zero difference to you grouping unless the trigger has poor mating faces and requires lube, never seen this on a HW100 though, and I have serviced well over a fifty.
Have you checked-
Barrel cleaned or does it need cleaning.
Were the conditions actually the same, it only takes small wind eddy's along the flight path to change a group and quite often what you think is similar conditions can be very different.
Is it the same batch of pellets, check the numbers.
Could be you having an off day