It seems that the inscrutable east is really eluding you!
The best way to understand this concept is by going back to the predominant Latin attitude about the nature of fallen man as opposed to the predominant attitude of the east about the nature of fallen man.
There have been many, many discussions about the two ideas of Original Sin (or First Sin) but I must ask you in charity to please not be readily dismissive of this.
Over time there was a considerable gulf between the understandings, and I am not stating that one or another was better!
If you can imagine all of us claiming an Immaculate Conception I am sure you would feel a bit of revulsion at the thought. So just gather that in…and feel it for a moment. That is the Latin attitude leaning toward the general level of depravity of all of us…certainly an unworthy bunch!
In that sense, no one can conceive of the Blessed Virgin being just like us…so unworthy and prone to almost any level of sin at any time. So the idea of Mary’s conception being special grew in the minds of many.
In the east, the mother of God Mary is primarily a heroic figure, something like a Horatio Alger success story. She did not sin because of her gumption, her true grit. The right stuff.
In other words, her sinlessness is not a foregone conclusion. In fact she is certainly not the only one so blessed (John the Baptist is another). It is a function of her will in cooperation with the Grace of God showering down upon us like the rain.
If one thinks in these terms, the concept of immaculate conception is unnecessary and superfluous. She is endowed with the same conception of all of us…in a sinless state any woman would have been just like her, because they were similarly born.
Michael