Messages had read receipts off,grey ticks turned blue weeks

Hello,

Was chatting to girl who had read receipts off. She was answering my messages and messages maintained grey ticks even when she was replying. So it was definitely off. 

Month later after we had stopped contact. All messages read receipts turned blue. How is this possible given they were old messages and read receipts were previously off.

Thank you! 

Android / Chrome 105.0.0.0