calculus - Integral of x² from 0 to b - using archimedes sum of squares - Apostol's

I'm reading Apostol's Calculus book and in the first chapter is presented the way archimedes found the sum of the square and how it can be used to calculate the integral of $x²$. But I'm not able to follow some steps of the proof.

from the book:

We subdivided the base in $n$ parts each with length $\ \frac{b}{n} $, a typical point corresponds to $\frac{kb}{n}$ where $k$ takes values from $k = 1, 2, 3, ..., n$

We can construct rectangles from for each $k th$ point:

$Base = \frac{b}{n}$

$Height = (\frac{kb}{n})^2$

$Area = Base * Height = \frac{b}{n} . (\frac{kb}{n})^2$

$Area = \frac{b^3}{n^3}.k^2 $

If we sum all the rectangles, we get a bit more than the area under the curve $x^2$

$S_{big} = \frac{b^3}{n^3}.(1² + 2² + 3² + ... + (n-1)² + n²)$

If we can construct smaller rectangles, using $n-1$ points, we we get a bit less than the area under the curve $x^2$.

$S_{small} = \frac{b^3}{n^3}.(1² + 2² + 3² + ... + (n-1)²)$

So the real area under the curve $x^2$ is between the two areas:

$S_{small} < A < S_{big}$

After a bit of algebra we get that:

$S_{big} = \frac{b^3}{n^3} . (\frac{n^3}{3} + \frac{n²}{2} + \frac{n}{6})$

$S_{small} = \frac{b^3}{n^3} . (\frac{n^3}{3} - \frac{n²}{2} + \frac{n}{6})$

To prove that $A$ is $\frac{b^3}{3}$ he uses this inequalities:

$1² + 2² + 3² + ... + (n-1)² < \frac{n³}{3} < 1² + 2² + 3² + ... + n²$

But I don't understand where the $\frac{n³}{3}$ came from. And can't follow the proof

After taking the average of the two expression:

$\frac{(\frac{n^3}{3} + \frac{n²}{2} + \frac{n}{6}) + (\frac{n^3}{3} - \frac{n²}{2} + \frac{n}{6})}{2} = \frac{n^3}{3} + \frac{n}{6}$

So I understan where $\frac{n³}{3}$ came from, but why is $\frac{n}{6}$ is thrown away?