What comes to mind when I tell you that im charging a fee of $0.5$% when you buy flowers from me?
You can either think of it as: If I want to purchase $1000$ dollars worth of flowers I need to pay $\$1005$. So Im getting $\frac{\text{investment}}{1.005}$ worth of flowers.
$$\frac{\frac{\text{investment}}{1.005}}{\text{Price}}=\text{Quantity Purchased}$$ or you can think of it as If i purchase $1000$ flowers, i need to pay $5$ flowers and therefore receive only $995$ flowers. $$\frac{\text{investment}}{\text{Price}}\cdot 0.995=\text{Quantity Purchased}$$
I think that the two are both representations of a $0.5$% fee and therefore I don't understand why the two give different values. i.e. $\frac{1}{1.005} \neq1\cdot 0.995$. I thought it might have to do with the fact that one was taxing the investment while the other was taxing the flowers, but rearranging: $$\frac{\frac{investment}{Price}}{1.005}=\text{Quantity Purchased}$$ of course doesn't change anything.
Ive encountered this problem before, but because it was so small, i always thought it was a floating point error in computations.
Update: I figured out its a matter of paying $0.5$% more than you're getting and getting $0.5$% less than what you're paying. But why the two would be different is still confusing.
The first way of looking at it is right.
As for the second way, $1000 \neq 1.005 \times 995$. It's close, but $995 \times 1.005 = 999.975$. So with your $0.5 \%$ you would pay $999.98$ dollars for $995$ flowers, not $1000$ dollars. Basically, $0.5 \%$ of $995$ (obviously) isn't equal to $0.5 \%$ of $1000$.
I think your biggest mistake is to think that $QuantityPurchased$ is the same for both ways of looking at it. For the first way, $QuantityPurchased = 1000$, whereas for the second way $QuantityPurchased = 995$.
I'm sorry if it's not written down all too clear, but I hope this helps you getting rid of your confusion.