That's honestly a great question!
Today is the 8th day of the Novena to the Holy Spirit. I find it interesting that today, Friday, we call upon the Holy Spirit as the Spirit of Wisdom, and today is the day this fine young woman asked this question!
"Embodying all the other gifts, as charity embraces all the other virtues, Wisdom is the most perfect of the gifts. Of wisdom it is written 'all good things came to me with her, and innumerable riches through her hands.' It is the gift of Wisdom that strengthens our faith, fortifies hope, perfects charity, and promotes the practice of virtue in the highest degree."
I couldn't help but think of the Blessed Mother upon reading those words. Indeed, she is the Mother of Wisdom. Through her, the supreme goodness of God came to us in the form of Jesus, Her Son. Being our most favored and beloved intercessor, we are granted innumerable graces through Her Immaculate hands. She is our perfect example of faith, hope and love, and when we dedicate ourselves to following her sweet example, we find that our faith is strengthened, our hope solidified and our love purified. In following Our Lady, we cannot help but find new ways to practice virtue.
Did the Holy Spirit "HAVE" to alight upon the Blessed Mother? Did she really "need" grace to perfect anything she was lacking to enable her to go forth and help build the newborn Church?
Considering how much the Holy Spirit must love Our Lady, who is to say He simply didn't WANT to pour Himself out upon her? Who is to say He didn't wish to miraculously expand her already pure and loving heart in order to fit more of Himself within?
Point is, pouring Himself out to Mary gave Him great joy because He knew how much joy He was giving her. In deepening and expanding her already perfect capacity to love, He didn't diminish her previous state of perfection.
Think of it this way. Captain Rich Pants is the World's Richest Man. He is a bazillionaire, and he happily gives away much of his fortune to charity. Putting another $100 into his bank account isn't going to have people saying, "Oh my gosh - he wasn't the World's Richest Man before! NOW he's the richest!"
No. Both before and after the $100 transaction, he was the World's Richest Man. Even WITH all the checks to charity.
Now let's say we up the transaction to one bazillion dollars (whatever that is). Would this addition be any different than the $100?
No. Both before and after either transaction, Captain Rich Pants was and would be the World's Richest Man.
The same is true of the Blessed Mother. Instead of being a bazillioniare, though, she was blessed with the Jackpot of all Graces. Just because the Holy Spirit wanted to gift His beloved an extra blessing of graces doesn't diminish the fact that she was perfected in graces before His arrival.
Another perspective - My Mom's van works perfectly fine. She loves it. It gets her to where she needs to go and she's able to pick up her friends and do grocery shopping and such. However, if I ever came into some money and purchased her a Stretch Hummer (that somehow ran only on used vegetable oil to stave off environmental irresponsibility), I would hope no one would give me the stink eye because the van she's had for a few years suited her just fine.
A gift is a gift. The Holy Spirit poured His Gifts out upon them freely and lovingly. Just as it'd make me happy to give my Mom an awesome upgrade to her car, the Holy Spirit was happy to give Mary an even deeper perfected love of Him and His Church.