It is not Call Presentation. In CCMA (web administration) go to Configuration - Skillsets. In the table there is a column for agent selection. This has the choices (none of them are round robin, that is for simple hunt groups) Longest Idle Since Logon, Longest time in Idle since last status change, and longest total time since last ACD call.
The only way calls are selecting the same agent first is if the agents are "playing" the system by going in and out of Not Ready, or pressing and releasing their DN key, etc. They may be using some function on the phone to reset their timer.
As long as you have current patches for your release you should determine if this is really happening (agent report will show how many calls each agent is getting) and if there is a strange pattern start watching agents.