Not able to get my head around Marginal cost.

I understand the principle of diminishing marginal product, wherein as the number of workers increase the marginal/additional production in quantity of the product they bring will start to decrease and Marginal cost principle is based on that.

So, let's say:

#workers--#product--Fixed_Cost---Variable_Cost---Total_Cost---Marginal_Cost

0----------0-------100-----------0----------100

1----------40------100-----------80 --------180----------$2

2----------90-----100-----------160-------260---------$1.6

3---------120 -----100----------240-------340--------$2.67

4---------145-----100----------320-------420---------$3.2

Now, when the 1 worker is appointed, he produces 40 units of additional product and to produce these 40 units, it costs an additional $2 per unit.
Which mathematically intuitive as well, because $2 * 40 = $80, which is precisely the extra cost.(180-100=80)

When 2 workers are appointed, they together produce 90, we still pay extra $80, but the extra production is 50, thereby reducing the additional cost per unit to $1.6. Now, this $1.6 cost is the cost of producing the additional 50 units,(again mathematically no problems, $1.6*50 = 80, and this is again precisely the extra cost we incur) but does this mean that the prior 40 units are still being produced at $2 a unit.

If I add the two marginal cost $1.6 +$2 = $2.6, where as Total Cost per unit is a bit greater for producing 90 units ($260/90) = $2.89.