What's the difference between self-isolate within 10 days and 14 days?

It's a change that occurred based on more understanding of the virus.

People with covid are contagious for 10 days, with the first 5 days being the most contagious.
97% of the people develop covid symptoms by the 10th day.  An itty bitty bit on the 15th day.

The change from 14 days isolation to 10 days occurred in mid-November.
The change is really to help with lifestyle and the CDC freely admits this.  That reducing the time period that individuals are in quarantine does create a limited risk to the public but it helps in the sense that asymptomatic/presymptomatic people will be more app to stay for 10 days vs 14.

Also, the guideline allows testing at day 7.  If you test negative at day 7, you can live self-quarantine. 

14 days is what the CDC guidelines used to say, they changed it to 10 days probably about a month ago 

Covid takes 2-14 to incubate so the CDC recommends that you self isolate for 14 days if you were exposed to someone with covid. After 14 days and you don’t have symptoms then you don’t have covid. 

Well, most people refuse to self isolate for 14 days if they were exposed to covid so the CDC dropped it down to 10 days because most people are more likely to quarantine for 10 days. After 10 days and you show no symptoms, you’re likely to be covid free. Most symptoms show after 10 days.