Both plants and algae adapt to CO2 in their environment. To do this, they produces what’s call the Rubisco. The Rubisco is an enzyme that is used for carbon fixation. In a low CO2 environment, plants and algae will produces more Rubisco so they can fix carbon from the KH (-HCO3). When there is plenty of CO2, plants and algae no longer need the Rubisco so they simply get rid of it.
Problems show up when CO2 is unstable because algae are much faster to adapt to low CO2 than plants. So everytime your CO2 is changing, algae have a chance to outcompete the plants.
When I saw the enzyme RuBisCO mentioned, I remembered the first time I heard that word, which was at a meeting of the Atlanta Area Aquarium Association, during member Brian Revennaugh's great presentation on planted aquariums. If you'd been in the room with me, you probably could have seen the light go on over my head. Aha!
I've been a student of aquarium keeping, and especially planted aquariums, my whole life, and it is wonderful to be able to keep learning things about it.
Addendum: Brian R. adds some depth and precision in his post at the AAAA Forum discussion of this topic.