How does sparrow wallet calculates whirpool transactions fees for posmixes ( or for this purpose samurai whirpool ) ?

nostr:npub1az9xj85cmxv8e9j9y80lvqp97crsqdu2fpu3srwthd99qfu9qsgstam8y8 nostr:npub1qny3tkh0acurzla8x3zy4nhrjz5zd8l9sy9jys09umwng00manysew95gx or anyone? Always shows 28 sats/vB and in transaction preview it's 31.1 sats/vB