I have a thermometer sitting 10cm away that reads 25.3ºC. Yet my thermistors are reading 23.67ºC and 25.51ºC
10cm = 4", that's a light-year when measuring temperature.
Aside from that, what you have to realize is that, with "analog" electronics
[and as others have mentioned] there is always a wide tolerance on devices
and measurements. To get real accuracy, designers good to extreme limits
on selecting low-tolerance parts, like 0.001% resistors and stable voltage
references. Eg, the tolerance on your standard 3.3V regulator is probably 2%,
if not 5%, and then it varies all over the place with temperature.
IOW, saying temperature = 25.51ºC is almost meaningless for a ckt with
common thermistors and Arduino powered by a standard v.reg. You're
probably lucky to be within 2 degC.