Why is NY state taxing income earned in Georgia state?

Hello TT experts, 

Another question re. my son's taxes --- he goes to school in Georgia & got a campus job in 2025 & earned some income in Georgia. 
In the Summer of 2025 he got an internship with a company in NY state where he earned some money.  He lived in NY state for 84 days total the Summer of 2025. That company has withheld NY state tax as stated in his W-2 from that company. 
when I was filling out NY State IT-203 form, the NY State taxable income was calculated to be the Federal income minus the NY state standard deduction. 
The Federal income for my son includes money earned from the on-campus job in Georgia + the money earned during the summer internship in NY state.

While he was in NY State during the summer, he was not working that on-campus job because school was closed for the summer. IOW, the Georgia on-campus income & the NY State income are mutually exclusive.
Why is NY State taxing his income earned in Georgia? --- shouldn't the Georgia income be off-limits for NY State?
thank you in advance for your insights re. this question.