Have you ever come across this Hadith:
حدثنی سعید ابن یحیی: خدثنا ابی:حدثنا مسعر عن الحکم عن ابن ابی لیلی عن کعب ابن عجره (رضی الله عنه): قیل یا رسول الله! اما السلام علیک فقد عرفناه، فکیف الصلاة علیک، قال(ص) قولوا: اللهم صل علی محمد و علی آل محمد، کما صلّیت علی آل ابراهیم انک حمید مجید اللهم بارک علی محمد و علی آل محمد کما بارکت علی آل ابراهیم انک حمید مجید
The Prophet Mohammad was asked: "As for (how to say) Peace on you, we already know it, so (let us know) how to bestow blessings on you. He replied: " Say: O' Allah bestow blessings on Muhammad and Muhammad's Family ( or clan) as you bestowed blessings on Abraham's Family, Indeed you are all-laudable all-glorious, O' Allah bless Muhammad and Muhammad's Family as you blessed Abraham's Family, indeed you are all-laudable all-glorious.
This Hadith is narrated in Tafsir book of Sahih Bukhari (No.4797) interpreting the verse 56 of Surah Ahzab:
إِنَّ اللَّهَ وَ مَلائِكَتَهُ يُصَلُّونَ عَلَى النَّبِيِّ يا أَيُّهَا الَّذينَ آمَنُوا صَلُّوا عَلَيْهِ وَ سَلِّمُوا تَسْليماً
Indeed Allah and His angels bless the Prophet; O you who have faith! Invoke blessings on him and invoke Peace upon him in a worthy manner.
So with due respect, why are our Sunni brothers insisting on leaving out the part "وآل محمد" from صلوات and saying just "صلی الله علیه و سلم" ?