Entering water in the wintertime is usually much cooler than in the summer, so that creates two problems.
1. As the water flows through the dip tube to the bottom of the tank it will have a greater cooling effect on the hot water at the top of the tank. And,,
2. Since the water is colder to start with it takes longer for it to be heated.
3. Both of these factors will cause the heater to NOT produce the quantity of hot water it does in the summertime.