All of the philosophically intellectual people agree on one point that Bhagvan propagated by the Veda(S) is one and absolute. Still we see many variations of school of thoughts in Hinduism, each started by an incarnation of an associate of Bhagvan, or a great devotee etc. for example Madhvacharya is Vayu dev, Vallabhacharya is Agni dev, Ramanujacarya is Shesha, Chaitanya mahaprabhu is Radha-Krishna combined etc. and each of these personalities gave society a different school of thought. So who is the correct one? And if all of them are directly an incarnation of God or his associate then why did they decide to create such a scene of confusion in the society.
Well the answer is here, all of the people vary from each other, everyone have different mindset, intellect level, maturity level due to previous Samsakara(S). Thus it is nearly impossible that even if logically proved a statement will be accepted by all heartedly, Thus Bhagvan gave these various schools of thought so that one can follow a school of thought in accord to their understanding capability and not fall for any Adharma and practice Dharma.
In my experience all of the schools at some or other time match with the philosophy of Advaita, mainly propagated by Adi Sankaracharya, even Madhvacharya whose school of thought is seen to be the rival of Sankara’s school, agrees with the philosophy of Shankaracharya while writing the commentary of Bhagvad Gita’s 13th chapter’s last verse.
The statement about the school of thought of Advaita is my own view, one may choose to disagree with it.