Shipping Tax Calculation with two tax rates

I was wondering how the shipping taxes are calculated when the products have different shipping rates. Like in this example:

Produkt1 net: 20.15€ (10 % taxed)
Produkt2 net: 13.32€ (20 % taxed)

shipping costs S = 4.99€ (incl. Tax) (this was set by myself and is fixed)

woocommerce calculates S_net = 4.39€ the net shipping cost with 0.27€ taxes for the 10% part and 0.33€ taxes for the 20% part.
so: 4.39 + 0.27 + 0.33 = 4.99 €

However, this does not align with my calculations which are as follows:

ratio of 10% taxed products r10 = 20.15/(20.15+13.32) = 0.602
ratio of 20% taxed products r20 = 13.32/(20.15+13.32) = 0.398
S_net = S/(r10*1.1+r20*1.2) = 4.99 / (0.602*1.1 + 0.390*1.2) = 4.377974 ~4.38

What I am doing wrong? I know it is just by 1 cent off, but this has an impact when doing 1000 orders.