Simply, there was a period of ever strained communication from the fifth century through the seventh. Add to this that during this period Orthodox, Oriental Orthodox and Church of the East were all cementing their Christological language (how we describe Christ) and their impressions of what the other groups believed. Next time, let us get Saint Ephraim and some other Syrians to help us define our terms so we can endeavor to answer some of the questions Saint Isaac’s place in history makes us ask.
For a great read on Saint Isaac of Nineveh check out The Spiritual World of Saint Isaac the Syrian, written by Metropolitan Hilarion Alfeyev: http://www.amazon.com/Spiritual-World-Syrian-Cistercian-Studies/dp/0879077751/ref=sr_1_2?ie=UTF8&qid=1343898666&sr=8-2&keywords=st+isaac+the+syrian
Or, just read him in his own words: http://www.bostonmonks.com/product_info.php/products_id/635