Warm sun + 0 wind chill + being acclimatised to 5C or less. Same deal as people suffering from heat stroke in 29C when parts of the world regularly don't fall below that for months out of the year.
Also depends if it’s the end of summer or the end of winter. If it’s the end of summer I would freeze my balls off, if it is the end of winter I’m sweating my balls off. It’s that brown fat that makes a huge difference and you make a lot more of it with cold exposure, many adults barely have any of it during the summer.