Order is always reduced to higher order terms. I can give you intuitive reasoning. Suppose you have O(n + n^2). Then which part would play more important role in run time? n or n^2. Obviously n^2. Because where there n^2 you won't notice effect of n when n is increased or decreased.
As example,
let n = 100, then n^2 = 10000
means n is 0.99% and n^2 is 99.01% of total running time.
What would you consider for runtime?
if n is increased then this difference is clearer.
I think you understand now,
