Looks like the codes are a bit random with how they apply discounts.
Terms and Conditions:Special available for bookings made by 28 October 2018 or until sold out. You are entitled only to the discount associated with the code in your email. The voucher code in this email may entitle you to a 31%, 21%* or 13%* discount off selected parking products. To be eligible to receive your discount, the voucher code must be used in the Voucher Code field on the booking site. Limited to the first 6,000 bookings. Must be booked online, at least 1 hour prior to arrival. Special not to be used in conjunction with any other offer. Valid for parking up until 30 June 2019. Brisbane Airport Parking terms and conditions apply, and can be found at www.bne.com.au/parking.