Hello,
SAM assumes that the system is always connected to the grid, and reduces the AC "System power generated" to account for the require tracking power. You can see this by making a copy of your case (click Duplicate on the Case menu), and setting Single tracker power (fraction of DC nameplate) on the Array page to zero. Then run a simulation, and on the Time series tab of the Results page, compare the "AC gross" and "System power generated" values of the two cases to see the impact.
Unfortunately, we do not have documentation of the HCPV model aside from the information in the Help system, and content of discussions on this forum.
Best regards,
Paul.