@LCPete I've no idea as to your level of knowledge with fish, so forgive me if this is already understood, but in case it is not, it is very important in understanding the significance of temperature.
You are correct that the temperature range on reliable sites is based upon observation in the habitat of the species, and often over a year. The suggestion one frequently encounters that there is some considerable variation in temperature diurnally and seasonally is not truly accurate in the true tropical regions, such as the Amazon basin, as in most cases the temperature varies by no more than a few degrees if that. But variation is not the same thing as permanent.
Ranges for a species on reliable sites are intended to be read as the range within which the fish should manage, but not exclusively at the lower end or the higher, but more in the middle. In the example with the two cory species, the ranges of 21-27C and 20-26C respectively mean that the fish will tolerate (using Ian's words here) the extremes for short periods but should be maintained long-term at 24-25C.
One might wonder why this matters. Fish are ectotherms; they do not generate internal heat but depend upon the temperature of the water in which they live to provide the heat necessary to live. Temperature is a prime driving force in an animal's metabolism, and each species has evolved to function in a specific temperature. Some species have much greater variability than others. But their metabolic functions need "x" warmth to work, even though the fish may manage/tolerate temporary periods outside this preference.