Since this question arises from time to time in relation to the Extraordinary Form, here is a response published in American Ecclesiastical Review, 1911.

Qu. Comparing the text of the Missale Romanum, Mechl. Typis H. Dessain, MCMIX, with the text of the Graduale Sacrosanctae Rom. Ecclesiae, Typis Societatis S. Joannis Evang. Romae, Tornaci, 1908, I notice a discrepancy in the Introit and Communio for the Feast of St. Stephen, Protom. The Missal reads: “Sederunt principes,” etc. (Introit); and: “et ne statuas illis hoc peccarum” (end of Communio). The Graduale reads: “Etenim sederunt,” etc. To the Communio it adds the words “quia nesciunt quid faciunt.” Both editions claim to be “typical”. Which formula is correct?
Ig. Forster, O.S.B.

Resp. Both formulas are, at present, correct. A decree S.R.C. dated 7 August, 1907, officially sanctioning the Vatican Graduale, referred to the fact that some readings found in it differed from those found in the typical Missal:

Some readings not found in the present Missal text have been restored in order to secure a better restoration of the form of the chant. In an audience granted 14 March, 1906 ... the restoration of these readings was expressly approved and prescribed by the Supreme Pontiff, and is to be absolutely observed in future editions of the Graduale.

A previous decree (8 June, 1907) ordered that no change should be made in the verbal text of the Typical Missal of 1900:

No change is to be made in the words of the text or in the rubrics, which therefore must be reproduced without modification, as in the last typical edition.

Doubtless the Missal text will ultimately be made to conform with that of the Graduale; and it was probably thought inadvisable either to delay the publication of the reformed Graduale by awaiting a similar reform of the Missal, or to cause additional expense to churches in requiring an immediate reform of the Missal text. Altogether, there are nearly two hundred variations between the present typical Missal texts and the present typical Graduale texts. The conflict between what the celebrant recites and what the choir sings does not, however, constitute an unpleasant or noticeable liturgical variation, since the celebrant reads the texts in a low voice, while the choir sings them in a clear and audible voice. For many centuries the celebrant did not even read the texts which the choir sang, although the present rubrical directions require him to do so.
H. T. H. (Source)