I don't know about the US but here in GB there are various angles to go on.
The first is the breeding cycle, which affects female horses. Females perform much worse from November to March than April to October because nature dictates the breeding cycle. What happens is that it is desirable for northern hemisphere mares to foal in the late winter / spring so that the foals have a better chance of survival.
The cycle is based on the amount of light that enters the horses' eye. In the late autumn / winter months this is reduced and affects the females performance during those periods.
In the summer months it is often said that "females perform better because they like the sun on their backs", and to a degree they do perform better but it's not because they like to top up their tan.
Another angle is that cold weather can affect horses that may appreciate colder air. Horses with accessories such as cheek pieces, tongue ties, wind operations may be able to take in more oxygen that would be more prevalent in colder conditions.
Sweat and it's affect on weight is another angle, as is - in GB - winter clothing allowance. The racing authorities allow jockeys to claim a free 2lb clothing allowance, whereas on other days it may be 1lb. Some believe that 1lb or 2lb difference does affect horse performance and jockeys and connections may choose to exploit this allowance.
Finally, winter climates can affect surfaces and draw biases. This may be temporary but it is definitely evident but only when the smart jockeys realise what is currently going on. Most jockeys will just stick to trainer instructions, which could be based on year round 'perceived' bias and thus be false.
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