For many generations, the commonly available siddurim in both Ashkenaz and Sfard were created by publishers. The true Sephardic nuschaos are far more reliable as coming from a reputable source, but those in the Ashkenazic realm, including the contemporary Ashkenaz and Sfard lack this feature. There are siddurim that are based on kabala from the Ari Zal, and these also include many differences. Ultimately, the base siddur for all of these Ashkenazic nuschaos is the Roedelheim Siddur that was published by Rav Volf Heidenheim (talmid of Rav Nosson Adler, and chaver to the Chasam Sofer). he published his siddur 8 times, with modifications in each subsequent edition. It was probably republished after his death, but i don’t know if there were changes.
Nusach is a complex issue, and there are many, many nushchaos, with variations in many of them. It is not uncommon for combinations between features of one nusach and those of another, with such traditions occurring in the Litvishe velt just as in the Chassidishe velt.