Among those who are curious about this topic, the various differences between the propers in the Missal and the propers in the Graduale Romanum have been a subject of curiosity. Steven van Roode, who typeset the Simple English Propers, posted on the MusicaSacra forum a beautiful summary of all the research that has gone into account for the differences that you will find, for example, in the OCP pew aide vs. the propers in the SEP. These paragraphs nicely sum it up.
OCP clearly prints the propers as they appear in the Missale Romanum:
Ant. ad communionem Ps 30,20 Quam magna multitúdo dulcédinis tuae, Dómine, quam abscondísti timéntibus te.SEP however uses the propers as they appear in the Graduale Romanum:
Vel: Mt 5,9-10 Beáti pacífici, quóniam fílii Dei vocabúntur. Beáti qui persecutiónem patiúntur propter iustítiam, quóniam ipsórum est regnum caelórum.
Ps 70, 16-18 Domine, memorabor iustitiæ tuæ solius: Deus, docuisti me a iuventute mea, et usque in senectam et senium, Deus, ne derelinquas me.The Apostolic Constitution Missale Romanum (April 3, 1969) explains why the Missal propers are sometimes different from the Graduale propers:
Anno A: Mt 16, 24 Qui vult venire post me, abneget semetipsum: et tollat crucem suam, et sequatur me.
Quod reliquum est, licet textus Gradualis Romani, ad cantum saltem quod attinet, non fuerit mutatus, tamen, facilioris intellectus gratia, sive psalmus ille responsorius, de quo S. Augustinus et S. Leo Magnus saepe commemorant, sive antiphonae ad introitum et ad Communionem in Missis lectis adhibendae, pro opportunitate, instaurata sunt.So, the Missal propers are intended for read Masses, whereas the Graduale propers are intended for sung Masses. That's why SEP adheres to the propers as they appear in the Graduale Romanum.
Even though the text of the Roman Gradual, at least that which concerns the singing, has not been changed, still, for a better understanding, the responsorial psalm, which St. Augustine and St. Leo the Great often mention, has been restored, and the Introit and Communion antiphons have been adapted for read Masses.