I'll quote Bruce Metzger's textual commentary.  This explains some of variation, but not the variation in translation.  Key is how to translate the aorist active imperative τήρησον and the dative ἐν τῷ ὀνόματί.  

1) *power* is a supplied word not in the Greek text, apparently taking the dative as instrumental.  

2) *faithful* in the alternate translation is also supplied.  It's the result of translating the dative as place/location.  


Senses in the New Testament

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Senses in the New Testament

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>       17:11      ᾧ δέδωκάς μοι {B}

> The reading that best accounts for the origin of the others has also the strongest attestation: the difficulty of ᾧ (which is read by 𝔓60, vid א A B C K L W Δ Θ Π Ψ 054 f  f  28 565 700 Byz Lect) prompted some copyists to replace the dative (which is attracted to the case of the antecedent) with the accusative ὅ (D* X 2148 al) or with the plural οὕς (Db 892vid 1009 vg goth eth geo2 al). The latter correction could also have been prompted by the recollection of ver. 6 or the statement in 18:9. The omission of one or more clauses from several ancient witnesses (𝔓66* ita, , , , ,  syrs copach2) may be due to the difficulty of the original reading, or it may be accidental.

      17:12      ᾧ δέδωκάς μοι, καί {B}

> See the comment on ver. 11.

> -- 
Metzger, B. M., United Bible Societies. (1994). *A textual commentary on the Greek New Testament, second edition a companion volume to the United Bible Societies’ Greek New Testament* (4th rev. ed.) (p. 213). London; New York: United Bible Societies.


  [1]: https://i.sstatic.net/ivVz7.png
  [2]: https://i.sstatic.net/93ac0.png