עֵדוּת | בִּֽיהוֹסֵף שָׂמוֹ בְּצֵאתוֹ עַל־ אֶרֶץ מִצְרָיִם
In Tehilim 81 we see Yosef spelled with an extra Hai? What is the reason for this?
The Gemara (Sotah 10b) says that this was his reward for resisting the advances of Potiphar's wife and thereby sanctifying Hashem's name.
Elsewhere (ibid. 36b) it also explains that the addition of this letter (with the extra spiritual powers it conferred on him) enabled Y(eh)osef to master the seventy languages that he needed to know in order to be able to function as viceroy of Egypt.
Because some names can be analyzed, or re-analyzed as theophoric names. A theophoric name is one which contains the name of Hashem. Often, these names begin with Yeho- or end with -yahu, and Yo can be a shorthand for it. It is possible that Yosef was not initially intended as such a theophoric name (depending on what "ushmi Hashem lo nadati lahem" means). I've claimed that Yehoshua was the first such theophoric name. Depending on the original intention, this is either a revealing of the initial meaning (Yosef Hashem li ben acher) or a reanalysis (Yosef as Yosef Hashem) in a poetic context in Tehillim.
For more in-depth discussion of this, see here: http://parsha.blogspot.com/2006/07/parshat-shelach-earlier-theophoric.html and earlier, in the post and comment section here: http://parsha.blogspot.com/2006/06/late-parshat-shelach-names-of-spies-in.html