How does Valve compute market fees?

[Wok]

I have a profile background for Spelunky.

Based on my understanding, the formula to compute the fee is:
max(0.01, 0.05*x)+max(0.01, 0.10*x)
where x is the price without any fee, i.e. the money that the seller receives.

If I want to sell my item for x=0.19€, then the buyer has to pay 0.21€, so the fee is 0.02€.
According to the formula, it should be 0.029€. I find it dubious that Valve truncates the value instead of rounding it to 0.03€.

If I want to sell it for x=0.33€, then the buyer has to pay 0.37€, so the fee is 0.04€.
According to the formula, it should be 0.0495€. Once again, I find it dubious that Valve truncates the value instead of rounding it to 0.05€.

If I want to sell it for any price between 0.34€ and 0.39€, then the fee is still 0.04€! According to the formula, it should be either 0.05€ (truncation), or worse 0.06€ (rounding).

If I want to sell it for x=0.40€, then the the fee is 0.06€, as expected.

If I want to sell it for x=0.49€, then the the fee is still 0.06€ (the buyer pays 0.55€), but it should be 0.0735€, so at least 0.07€ (truncation or rounding).

Is there some explanation somewhere? I cannot find much help in the FAQ.

The FAQ states that:

The Steam Transaction Fee is calculated based on the item cost

I assumed the "item cost" is what the seller receives, because that is how you compute taxes, e.g. VAT.

Cross-posting there.