Siege mechanics in manual wrong?

[Xietor]

I did a test with Call of the Winds. It summons 1 str 11 leader and 20 str 5 hawks. The manual says flying units get a plus 1 added to their siege total. Yet the 21 flying units only do a total of 6 siege power to an empty castle each turn.

The bare min. 21 flying units could possibly do if the formula is correct is 21(and that is if they are str 0).

1 hawk should do: 5 x 5 = 25/100 =.25 add the flying bonus of 1 =1.25

1.25 x 20= 25

great hawk str 11

11x11 =121/100=1.21 plus flying bonus=2.21

Call of winds correct siege bonus under the manual formula is
27.21 not 6.

Any ideas why it is only doing 6 siege damage per turn rather than 27.21?

Edited for math error pointed out below(thx).