Swami Vivekananda has said that avataras are always different from ordinary gurus even if they have attained the highest state.
Complete Works/Volume 5/Questions and Answers:
Q.—Cannot the spiritual teacher make the aspirant fit?
A.— An Avatâra can. Not an ordinary Guru.
Q.—Is there no easy way to liberation?
A.—"There is no royal road to Geometry"—except for those who have been fortunate enough to come in contact with an Avatara. Paramahamsa Deva used to say, "One who is having his last birth shall somehow or other see me."
Q.—What is the meaning of "क्षणमिह सज्जनसंगतिरेका"—"Even a moment's association with the holy ones serves to take one beyond this relative existence"?
A.—A fit person coming in contact with a true Sadhu attains to liberation. True Sadhus are very rare, but their influence is such that a great writer has said, "Hypocrisy is the tribute which vice pays to virtue." But Avataras are Kapâlamochanas, that is, they can alter the doom of people. They can stir the whole world. The least dangerous and best form of worship is worshipping man. One who has got the idea of Brahman in a man has realised it in the whole universe. Monasticism and the householder's life are both good, according to different circumstances. Knowledge is the only thing necessary.
He is saying that Avataras can give liberation so why can't ordinary realized gurus do that?
One who can alter the doom of people is the Lord. No sadhu, however advanced, can claim this unique position.
Why no sadhu can claim this unique position? Speaking from Advaita perspective, the moment you get realized, you become aware of your true nature. You realize that you are no different from indivisible Brahman and that is what Avatara is: God in human form.
So what exactly is the difference between avataras and normal gurus?