Mary may not have been aware that She was sinless, it may have been considered pride for Her to think so. Be advised that the following is from a Pro-Medjugorje website which is currently under investigation from the Vatican, but it gives the best explanation I’ve seen:
"Some non-Catholics may also have difficultly understanding the Co-Redemptive process, considering Mary to be only some kind of a birthing vessel, even capable of possessing the same sins and sinful desires as any other woman.
The Ark of the Covenant was so Holy, that God gave specific instructions as to how It must be built. Every single detail; from the wood that must be used, to what metals to incorporate, how to carry It, how to hold It, what was to be placed in It, etc. God even warned; if It starts to tip or fall don’t touch It. We all know what happened when the Ark was being carried by the Israelites on an ox-cart and began to tilt, an Israelite put his hand on It to stabilize It, and God instantaneously struck him dead. God warned not to touch It, and He immediately defended His Vessel with severe capital punishment. God doesn’t give these exacting instructions in the Old Testament only once, but He gives them twice. The Ark possessed such incredible power, that armies who carried It before them, won their victories. When King David recovered the Ark he danced every seven steps proclaiming his joy at getting the Ark Back. He was even chastised by his wife who said he was making a fool out of himself. He told her that she didn’t understand the significance. We got the Ark back! He knew there was power there!
And if there was so much power with the Ark because It contained objects that were touched by the very Finger of God (the Jewish Decalogue known as the Ten Commandments, on two tablets that God inscribed with His Finger), how much more power is there in the Container that enclosed the actual Divine Holy Finger of God? And not just the Finger, but the Hand of God, and not just the Hand, but the entire Body of God? And how much more Holy of a Vessel would be required to carry all that Divinity? And how much more exacting and perfect construction would be required to build such a Faultless Holy Vessel? And that is what Mary is, God’s most Perfect Ark, the bearer of Himself."
DefendingMedjugorje.com/Pages/Antagonist.htm
God Bless