I was trying to gild Herald Cape

with Metal Buttons

The both have only one gilding slot for Sewing. My sewing is at 240, so, according to wiki, I must have max chance for slotting

The chance for slotting must be 0,9 * 0,3 = 0,27 (%)
I also have tailor's credo completed, so the final chance must be 0,27 + 0,05 = 0,32 (%). Or at least more the 27%
But when I try to gild, the chance shown is 27%

Am I missing something? Does gilding work in a diffirent way?