Hello Martin! I'm Jen, an independent advisor and a fellow Microsoft user like you. I'd be happy to help you out with this issue.
Could you please check if your data in column A are formatted as numbers? Try changing the values with dot as decimal separator (example: for A1, make it 7.42) and see if it makes any difference.
Meanwhile, you might want to check out this video link which discussed about using PHStat to solve a hypothesis test for single sample t-test for the mean.
https://www.youtube.com/watch?v=Qq7vRipF1x0
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
Hope this helps you.
If you have any questions, please let me know and I'd be glad to assist you further.
Thank you and Best Regards!
Jen :)