You are conflating 2 things:
with a normal distribution 95% of values are in the range mean +/- 2 standard deviations
given the sample mean, what is a confidence interval for the true mean. Excel is telling you that there is a 95% chance that the true mean is in the range 236 to 287. This confidence interval will get tighter as the number of samples increases (for more detail on this, lookup the central limit theorem)
The excel documentation for the confidence function has some further explanation