Hi and thanks for letting me know - the mean and SD are currently coded to match the formatting of the y-axis, which should refer back to the formatting applied to the measure in the data model. Can you please confirm if your measure has any formatting applied to it in the Desktop ribbon?
Edit: I'm not particularly happy with this current implementation BTW and will de-couple the formatting of the tooltip from the y-axis in a future (probably next) release.
This has been resolved as part of the locale work done in
#63.