Looking at their CItibank Rewards T&Cs below, looks like they could get away to change the price of "individual items":
The file below can be access from their Velocity/Krisflyer redemption page at Terms and Conditions:
https://www.citibank.com.au/global_..._Program_TandCs_for_Classic_Gold_and_Plat.pdf
General
6.1 We may decide not to offer one or more of
the Rewards Programs. If we do this we will
give all participants in the relevant Rewards
Program at least 90 days written notice
before we terminate the Rewards Program,
including whether any points may be
transferred to another Rewards Program or
must be redeemed before a particular date.
6.2 We may vary these Rewards Terms and
Conditions from time to time. We will notify
you of a change in the following ways:
• If we change the rate at which all Points
in the program are generally earned or
converted we will give you 90 days written
notice of that change;
• We will give you 30 days written notice
of other changes to these Terms and
Conditions;
• If we change the Point value of individual
goods or services in the program to reflect
changes in the price charged by our
suppliers for those goods or services, or if
a particular Reward is unavailable we will
notify you on our Rewards website at the
time you redeem your Reward.
...
11. Airline Loyalty Programs
11.1 If your account allows you to redeem Points
for Airline Loyalty Points, the types of
Airline Loyalty Points that we offer may
change over time because of decisions
taken by the airline. Please refer to the
current Rewards catalogue or the Rewards
website for details of which Airline Loyalty
Points are currently offered.
11.2 You must be a member of the Airline
Loyalty Program in which you wish to
claim Airline Loyalty Points and have
advised us of your Airline Loyalty Program
membership number prior to any such
redemption.
11.3 If you are a member of the Airline Loyalty
Program, you are subject to the terms and
conditions of the Airline Loyalty Program.
1211.4 If you redeem Points for Airline Loyalty
Points, the Airline Loyalty Points may
take up to 28 business days to appear in
your Airline Loyalty Program membership
account.