SELECT Top 10 and order by Results vary every time ( different results every time i run)

EX:

Select top 100 col1, col2, col3,xyz.col4
from ABC
JOIN XYZ
Order col3


i have that query ran at 10AM and i ran the same query at 10:15am
the results were different...
Y?


Oderby Clo3 is not uinique


I had to put Unique column (PK column, INDEX Column) to get the same results everyTime

This is just my exp:
i see online some parallel processing,. ,Maxdop etc things