so'jhin and da'covale are two different things. One is made da'covale as a punishment and apparentyu kept for lowly menial tasks. Descendants of someone punished this way are often forced into service to the throne as da'covale too (it doesn't seem to be always the case, as some like Karede and the Seeker were called hereditary da'covale - that'd be redundant if they all are), but can eventually rise very high (as Karede and Selucia, for example) and even be named so'jhin (as happened to Selucia)

The So'jhin status is rather an honor. They're the throne higher servants. Thy can be anyone the Empress wishes to raise to that status, including hereditary da'covale.. It's not an hereditary honor (the herediatry honor is to be raised to the Blood or High Blood). It's not because her mother was at the top of the Seanchan military fleet and so'jhin that her children have the same honor. It's based on merit, and earned or not at every generation.

Egeanin was a free woman because her mother was so'jhin but not an hereditary da'covale. She was also a free woman.

Seanchan slavery is a peculiar thing. They don't do or see things like the Europeans/Americans did, nor see their slaves as sub-humans (with the exception of damane). Everyone in Seanchan serves the Throne, even the Empress herself. Those who fail (by betrayal, crime, incompetence etc.) become part of a caste that doesn't have anymore the freedom to choose how they will serve. It's a lot more similar to slavery and serfdom in ancient Asia, and even to some aspects of the caste system in India.

The whole Seanchan society is divided into a hierarchy of duties and status. A High Lord has no more choice than a da'covale but to fulfill the duties the Throne asks of him, and like them he is born into the status. The difference is that a High Lord has a high status, and with it comes a more complicated (though more comfortable) life and is even under greater pressure of competence. Even Tuon herself is hardly "free". She has no choice but to be who she is. Her whole life has been programmed for her. As everyone has those duties to the Throne and the Empire, the Seanchan don't really see how da'covale are so different. To them are just people who failed in their duty to the Empire (or whose bloodline is tainted with crime against the Empire) - or in the case of damane women they perceive can't be trusted because of their nature - and who have to be forced to do what everyone else in Seanchan is expected to do freely.