Air carrier Captain reported excess CO2 readings during climb which was resolved by the flight crew. Subsequently the flight crew discovered a discrepancy between the declared amount and the actual amount of dry ice loaded amount.

Narrative

We were under the dry ice exemption and carrying 1013.4 KG of dry ice. On climb out passing approximately FL240 our CO2 monitors started going off. There were discrepancies between our two monitors; reading anywhere between 0.4 and 1.9. As we continued our climb; we turned on the spare monitoring unit. It was at this time all three were alerting us to levels well above 0.5. We went on oxygen; leveled at FL380 and consulted the QRH. Our Oxygen table allowed us approximately 4 hours of oxygen. We alerted Dispatcher as to our situation. We elected to descend to FL200 in hopes of getting better airflow. At FL200 our bleed air duct pressure was 40 psi vs 30 psi at FL380. This brought our CO2 levels down to 0.4 and allowed us to go off oxygen. It was at this time we elected to continue to ZZZ vs diverting in ZZZ1.One of two things... either insufficient pack airflow for the amount of dry ice onboard; or there was way more dry ice onboard than was declared. More careful auditing of the actual hazardous materials (dry ice) on board our aircraft. The fact that we can be dispatched with 907 KG dry ice without an exemption (even with a single pack) is one thing. We had a 'declared' amount of 1013 KG of dry ice; two packs and had major CO2 level issues! This shows me that there might have been a problem with the actual amount of dry ice that was onboard. With this; I find it absurd that [the company] has now also changed our dry ice exemption to a total limit of 3402 KG of dry ice; approximately 1;000 more KG's than it use to be. I find this amount unsafe!